Backport dotnet-install script from a941e34

Partial backport of #9761

Update `build-tools/xaprepare` to improve reliability when
running `Step_InstallDotNetPreview`.  `Step_InstallDotNetPreview`
will cache e.g. `dotnet-install.sh` into `$HOME/android-archives`,
but there was no logic to verify that it was still *valid*.

We've been seeing some recurring build failures in **macOS > Build**
such as:

	Downloading dotnet-install script...
	Warning: Using cached installation script found in '/Users/builder/android-archives/dotnet-install.sh'
	Discovering download URLs for dotnet SDK '10.0.100-preview.2.25102.3'...
	Downloading dotnet archive...
	dotnet archive URL https://dotnetcli.azureedge.net/dotnet/Sdk/10.0.100-preview.2.25102.3/dotnet-sdk-10.0.100-preview.2.25102.3-osx-arm64.tar.gz not found
	Downloading dotnet archive...
	dotnet archive URL https://dotnetcli.azureedge.net/dotnet/Sdk/10.0.100-preview.2.25102.3/dotnet-dev-osx-arm64.10.0.100-preview.2.25102.3.tar.gz not found
	Downloading dotnet archive...
	Warning: Failed to obtain dotnet archive size. HTTP status code: InternalServerError (500)
	Downloading dotnet archive...
	Warning: Failed to obtain dotnet archive size. HTTP status code: InternalServerError (500)
	  Error: Installation of dotnet SDK '10.0.100-preview.2.25102.3' failed.

	Step Xamarin.Android.Prepare.Step_InstallDotNetPreview failed
	System.InvalidOperationException: Step Xamarin.Android.Prepare.Step_InstallDotNetPreview failed
	   at Xamarin.Android.Prepare.Scenario.Run(Context context, Log log) in /Users/builder/azdo/_work/8/s/xamarin-android/build-tools/xaprepare/xaprepare/Application/Scenario.cs:line 50
	   at Xamarin.Android.Prepare.Context.Execute() in /Users/builder/azdo/_work/8/s/xamarin-android/build-tools/xaprepare/xaprepare/Application/Context.cs:line 488
	   at Xamarin.Android.Prepare.App.Run(String[] args) in /Users/builder/azdo/_work/8/s/xamarin-android/build-tools/xaprepare/xaprepare/Main.cs:line 155

Indeed, [`dotnet-sdk-10.0.100-preview.2.25102.3-osx-arm64.tar.gz`][0]
no longer exists on <https://dotnetcli.azureedge.net>.

The problem, though, is that .NET changed the CDN that is used in the
past month, and that's not the correct URL.  A newer `dotnet-install.sh`
reports:

	% bash "…/dotnet-install.sh" "--version" "10.0.100-preview.2.25102.3" "--install-dir" "/Volumes/Xamarin-Work/src/dotnet/android/bin/Release/dotnet" "--verbose" "--dry-run
	…
	dotnet-install: Link 0: primary, 10.0.100-preview.2.25102.3, https://builds.dotnet.microsoft.com/dotnet/Sdk/10.0.100-preview.2.25102.3/dotnet-sdk-10.0.100-preview.2.25102.3-osx-x64.tar.gz
	dotnet-install: Link 1: legacy, 10.0.100-preview.2.25102.3, https://builds.dotnet.microsoft.com/dotnet/Sdk/10.0.100-preview.2.25102.3/dotnet-dev-osx-x64.10.0.100-preview.2.25102.3.tar.gz
	dotnet-install: Link 2: primary, 10.0.100-preview.2.25102.3, https://ci.dot.net/public/Sdk/10.0.100-preview.2.25102.3/dotnet-sdk-10.0.100-preview.2.25102.3-osx-x64.tar.gz
	dotnet-install: Link 3: legacy, 10.0.100-preview.2.25102.3, https://ci.dot.net/public/Sdk/10.0.100-preview.2.25102.3/dotnet-dev-osx-x64.10.0.100-preview.2.25102.3.tar.gz

Note the different domain, <https://builds.dotnet.microsoft.com>!

Update `Step_InstallDotNetPreview` to try to install .NET potentially
*twice*: the first time using the cached `dotnet-install.sh`, and
*if that fails*, it tries again after downloading a *new*
`dotnet-install.sh`.

Hopefully this will fix the build failure on this machine!

[0]: https://dotnetcli.azureedge.net/dotnet/Sdk/10.0.100-preview.2.25102.3/dotnet-sdk-10.0.100-preview.2.25102.3-osx-arm64.tar.gz

Author: jonpryor

build-tools/xaprepare/xaprepare/Steps/Step_InstallDotNetPreview.cs

@@ -21,7 +21,8 @@ protected override async Task<bool> Execute (Context context)
 			var dotnetPath = Configurables.Paths.DotNetPreviewPath;
 			dotnetPath = dotnetPath.TrimEnd (new char [] { Path.DirectorySeparatorChar });
 
-			if (!await InstallDotNetAsync (context, dotnetPath, BuildToolVersion)) {
+			if (!await InstallDotNetAsync (context, dotnetPath, BuildToolVersion, useCachedInstallScript: true) &&
+					!await InstallDotNetAsync (context, dotnetPath, BuildToolVersion, useCachedInstallScript: false)) {
 				Log.ErrorLine ($"Installation of dotnet SDK '{BuildToolVersion}' failed.");
 				return false;
 			}
@@ -77,17 +78,18 @@ protected override async Task<bool> Execute (Context context)
 			return true;
 		}
 
-		async Task<bool> DownloadDotNetInstallScript (Context context, string dotnetScriptPath, Uri dotnetScriptUrl)
+		async Task<bool> DownloadDotNetInstallScript (Context context, string dotnetScriptPath, Uri dotnetScriptUrl, bool useCachedInstallScript)
 		{
 			string tempDotnetScriptPath = dotnetScriptPath + "-tmp";
 			Utilities.DeleteFile (tempDotnetScriptPath);
 
 			Log.StatusLine ("Downloading dotnet-install script...");
 
-			if (File.Exists (dotnetScriptPath)) {
+			if (useCachedInstallScript && File.Exists (dotnetScriptPath)) {
 				Log.WarningLine ($"Using cached installation script found in '{dotnetScriptPath}'");
 				return true;
 			}
+			Utilities.DeleteFile (dotnetScriptPath);
 
 			Log.StatusLine ($"  {context.Characters.Link} {dotnetScriptUrl}", ConsoleColor.White);
 			await Utilities.Download (dotnetScriptUrl, tempDotnetScriptPath, DownloadStatus.Empty);
@@ -173,7 +175,7 @@ string[] GetInstallationScriptArgs (string version, string dotnetPath, string do
 			return args.ToArray ();
 		}
 
-		async Task<bool> InstallDotNetAsync (Context context, string dotnetPath, string version, bool runtimeOnly = false)
+		async Task<bool> InstallDotNetAsync (Context context, string dotnetPath, string version, bool useCachedInstallScript, bool runtimeOnly = false)
 		{
 			string cacheDir = context.Properties.GetRequiredValue (KnownProperties.AndroidToolchainCacheDirectory);
 
@@ -183,7 +185,7 @@ async Task<bool> InstallDotNetAsync (Context context, string dotnetPath, string
 			Uri dotnetScriptUrl = Configurables.Urls.DotNetInstallScript;
 			string scriptFileName = Path.GetFileName (dotnetScriptUrl.LocalPath);
 			string cachedDotnetScriptPath = Path.Combine (cacheDir, scriptFileName);
-			if (!await DownloadDotNetInstallScript (context, cachedDotnetScriptPath, dotnetScriptUrl)) {
+			if (!await DownloadDotNetInstallScript (context, cachedDotnetScriptPath, dotnetScriptUrl, useCachedInstallScript)) {
 				return false;
 			}