Add Newtonsoft.Json ref in RepoTasks

Author: mthalman

eng/tools/RepoTasks/RepoTasks.csproj

@@ -15,6 +15,9 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <!-- Explicitly reference the desired version of Newtonsoft.Json to avoid getting an older version
+         from the dependency on NuGet.Packaging which would cause a source-built prebuilt. -->
+    <PackageReference Include="Newtonsoft.Json" Version="$(NewtonsoftJsonVersion)" />
     <PackageReference Include="NuGet.Packaging" Version="$(NuGetPackagingVersion)" />
     <PackageReference Include="Microsoft.Extensions.DependencyModel" Version="$(MicrosoftExtensionsDependencyModelVersion)" />
   </ItemGroup>