Try to fix another MSBuild CI deps error

/home/runner/.dotnet/sdk/5.0.408/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(1545): error MSB4018: The "GenerateDepsFile" task failed unexpectedly.
System.TypeLoadException: VTable setup of type System.Text.Json.Utf8JsonWriter failed
  at Microsoft.NET.Build.Tasks.GenerateDepsFile.WriteDepsFile (System.String depsFilePath) [0x00281] in <08e9b3849e7a4e0d9dcff0965e290a65>:0

Author: mhutch

Diff for: Mono.TextTemplating.Build.Tests/Mono.TextTemplating.Build.Tests.csproj

@@ -20,6 +20,7 @@
     and loads first, thereby breaking loading of MSBuild assemblies. Force-upgrade it.
     -->
     <PackageReference Include="NuGet.Frameworks" Version="6.3.1" />
+    <PackageReference Include="System.Text.Json" Version="6.0.6" />
   </ItemGroup>
 
   <ItemGroup>