[build] Add support for Visual Studio on win-arm64

Context: xamarin/yaml-templates#204
Context: dotnet/android#7471

Updates the VSMAN files generated for our .NET workload to support
Visual Studio on windows-arm64.

dotnet/generate-vs-workload.csharp

@@ -87,7 +87,7 @@ using (TextWriter writer = new StreamWriter (outputPath)) {
  foreach (var entry in platforms) {
  var platform = entry.Item1;
  var version = entry.Item2;
- writer.WriteLine ($" <WorkloadPackages Include=\"$(NuGetPackagePath)\\Microsoft.NET.Sdk.{platform}.Manifest*.nupkg\" Version=\"{version}\" />");
+ writer.WriteLine ($" <WorkloadPackages Include=\"$(NuGetPackagePath)\\Microsoft.NET.Sdk.{platform}.Manifest*.nupkg\" Version=\"{version}\" SupportsMachineArch=\"true\" />");
  }
  foreach (var entry in platforms) {
  var platform = entry.Item1;