Disable Xamarin projects on CI

dirs.proj

@@ -8,6 +8,7 @@
     <ProjectReference Remove="src/Markup/Avalonia.Markup.Xaml/PortableXaml/**/*.*proj" />
     <ProjectReference Remove="src/Markup/Avalonia.Markup.Xaml/XamlIl/**/*.*proj" />
   </ItemGroup>
+<!-- Disabled on CI because of ancient MSBuild project format
   <ItemGroup Condition="!Exists('$(MSBuildExtensionsPath)\Xamarin\Android')">
     <ProjectReference Remove="src/Android/**/*.*proj" />
     <ProjectReference Remove="samples/ControlCatalog.Android/ControlCatalog.Android.csproj" />
@@ -16,6 +17,7 @@
     <ProjectReference Remove="src/iOS/**/*.*proj" />
     <ProjectReference Remove="samples/ControlCatalog.iOS/ControlCatalog.iOS.csproj" />
   </ItemGroup>
+-->
   <ItemGroup Condition="!$([MSBuild]::IsOsPlatform('Windows')) OR '$(MSBuildRuntimeType)' != 'Full'">
     <ProjectReference Remove="src/Windows/Avalonia.Win32.Interop/Avalonia.Win32.Interop.csproj" />
     <ProjectReference Remove="samples/interop/**/*.*proj" />