Fix the VS2019 build.

xamarin-android Windows PR builds can't build, because of:

	error CS8032: An instance of analyzer System.Text.Json.SourceGeneration.JsonSourceGenerator cannot be created from C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\6.0.0\analyzers\dotnet\cs\System.Text.Json.SourceGeneration.dll : Could not load file or assembly 'Microsoft.CodeAnalysis, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.. [C:\a\_work\1\s\external\Java.Interop\build-tools\jnienv-gen\jnienv-gen.csproj]

Ignore the CS8032 warning/error, so that xamarin-android Windows
PR builds can *build*.

Author: jonpryor

Directory.Build.props

@@ -91,6 +91,20 @@
     <_RunJNIEnvGen Condition=" '$(JIBuildingForNetCoreApp)' != 'True' ">$(Runtime) "$(_JNIEnvGenPath)"</_RunJNIEnvGen>
   </PropertyGroup>
 
+  <!--
+    When building on a bot w/ VS2019:
+
+      warning CS8032: An instance of analyzer System.Text.Json.SourceGeneration.JsonSourceGenerator cannot be created from
+
+    however, with `$(TreatWarningsAsErrors)`=True, this becomes an error,
+    meaning we can't build on VS2019.
+
+    Ignore CS8032 so that we can build on VS2019.
+    -->
+  <PropertyGroup>
+    <NoWarn>$(NoWarn);CS8032</NoWarn>
+  </PropertyGroup>
+
   <!-- The net6.0 versions of these are stricter and require overloads not available in .NET Framework, so start with just .NET Framework -->
   <PropertyGroup Condition=" '$(JIBuildingForNetCoreApp)' != 'True' ">
     <AnalysisMode>AllEnabledByDefault</AnalysisMode>