Follow-up on updating maintenance-packages dependencies conditioned to only DotNetBuildSourceOnly != true

[carlossanlop]

This sdk deps flow PR from runtime failed because we shipped new package versions from dotnet/maintenance-packages for System.Buffers, System.Memory, System.Runtime.CompilerServices.Unsafe, System.Numerics.Vectors and System.Threading.Tasks.Extensions.

System.Memory in particular is failing in dotnet/roslyn due to not being able to find System.ReadOnlySpan, which is provided by the package. The underlying root cause is that msbuild is forcing the package version to downgrade. Until we get msbuild to update the package versions, roslyn cannot have those package versions updated.

The sdk deps flow PR is getting at least 2 patches to condition those deps new versions to only happen when not in source build. We will need to take those conditions eventually when the timing is right (maily after msbuild update flows to roslyn).

cc @ViktorHofer @ericstj