nuget-client repo uses the VMR package cache instead of repo local

[ViktorHofer]

All other repos in the VMR use a separate repo specific package cache.

[ViktorHofer]

cc @mthalman as this could result in build order issues with the now enabled parallel builds

[corngood]

@ViktorHofer I inadvertently 'fixed' this by adding <ItemGroup><EnvironmentVariables Include="NUGET_PACKAGES=$(ProjectDirectory)artifacts/sb/package-cache/"... to repo-projects/nuget-client.proj, while I was trying to get packages in consistent locations for dependency gathering (#4333).

this could result in build order issues with the now enabled parallel builds

I think that's exactly what I discovered and attempted to fix here:

NuGet/NuGet.Client#5756

I don't think it's related to parallel builds, but I think these missing packages were restored by previous repos when using a shared packages dir.

[ViktorHofer]

I think this already got fixed by NuGet/NuGet.Client#5749, cc @mmitche