Duplicate 'PackageReference' items found error when building in source-build

[MichaelSimons]

The following error occurs when building sourcelink in source-build with the latest toolset

sourcelink/artifacts/source-build/self/src/src/Microsoft.Build.Tasks.Tfvc/Microsoft.Build.Tasks.Tfvc.csproj : error NU1504: Duplicate 'PackageReference' items found. Remove the duplicate items or use the Update functionality to ensure a consistent restore behavior. The duplicate 'PackageReference' items are: Microsoft.Build 17.3.0-preview-22330-01, Microsoft.Build 17.3.0-preview-22330-01; Microsoft.Build.Tasks.Core 16.7.0, Microsoft.Build.Tasks.Core 16.7.0.

By looking at [Microsoft.Build.Tasks.Tfvc.csproj](https://github.com/dotnet/sourcelink/blob/main/src/Microsoft.Build.Tasks.Tfvc/Microsoft.Build.Tasks.Tfvc.csproj), you can clearly see the Microsoft.Build and Microsoft.Build.Tasks.Core PackageReferences are duplicated.