[CI] Grab 'dotnet-install.sh' script directly from GitHub. (#9699)

Our CI builds are frequently failing to download the
`dotnet-install.sh` script from the official .NET website due to an
`AuthenticationException`.

Instead, grab the `dotnet-install.sh` script directly from GitHub to
avoid the issue.

Author: jpobst

build-tools/xaprepare/xaprepare/ConfigAndData/Configurables.Unix.cs

@@ -15,7 +15,11 @@ partial class Defaults
 
 		partial class Urls
 		{
-			public static readonly Uri DotNetInstallScript = new Uri ("https://dot.net/v1/dotnet-install.sh");
+			// This is the "public" url that we really should be using, but it keeps failing with:
+			// AuthenticationException: The remote certificate is invalid because of errors in the certificate chain: RevocationStatusUnknown
+			// For now we'll grab it directly from GitHub
+			// public static readonly Uri DotNetInstallScript = new Uri ("https://dot.net/v1/dotnet-install.sh");
+			public static readonly Uri DotNetInstallScript = new Uri ("https://raw.githubusercontent.com/dotnet/install-scripts/refs/heads/main/src/dotnet-install.sh");
 		}
 	}
 }