[build] set $(DisableTransitiveFrameworkReferenceDownloads)=true (#216)

As we consume nightly .NET 8 builds, they sometimes depend on nightly
.NET 7 builds.

One error you can run into is:

	error NU1102: Unable to find package Microsoft.AspNetCore.App.Ref with version (= 7.0.11)
	error NU1102: Unable to find package Microsoft.WindowsDesktop.App.Ref with version (= 7.0.11)

For projects that are not even ASP.NET or Windows desktop apps!
To even be able to access these feeds, they would need to be an entry
within `NuGet.config` similar to

	<packageSources>
	  <clear/>
	  <add key="darc-pub-dotnet-aspnetcore-[SHA]" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-aspnetcore-[SHA]/nuget/v3/index.json" />
	  <add key="darc-pub-dotnet-windowsdesktop-[SHA]" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-windowsdesktop-[SHA]/nuget/v3/index.json" />
	</packageSources>

We don't currently track these packages, because we don't actually
use them.

The .NET SDK team has provided a setting to workaround this,
[`$(DisableTransitiveFrameworkReferenceDownloads)`][0], we have been
[using in xamarin/xamarin-android for some time][1].

Let's do the same here to avoid this problem as seen in 2b8ac12.

[0]: https://learn.microsoft.com/en-us/dotnet/core/project-sdk/msbuild-props#disabletransitiveframeworkreferencedownloads
[1]: https://github.com/xamarin/xamarin-android/blob/6768c731d327c8148c45304c895ca8987a9cc2f1/Directory.Build.props#L26-L27

Author: jonathanpeppers

Directory.Build.props

@@ -3,6 +3,8 @@
   <!-- Note: MUST be imported *after* $(Configuration) is set! -->
   <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <!-- Disables the transitive restore of packages like Microsoft.AspNetCore.App.Ref, Microsoft.WindowsDesktop.App.Ref -->
+    <DisableTransitiveFrameworkReferenceDownloads>true</DisableTransitiveFrameworkReferenceDownloads>
   </PropertyGroup>
   <Import
       Project="$(MSBuildThisFileDirectory)Configuration.Override.props"