Update MicrosoftBuildVersion to 17.12.50#6870

Nigusu-Allehu wants to merge 3 commits intorelease/7.0.xNuGet/NuGet.Client:release/7.0.xfrom

backport/pr-6866-to-release/7.0.xNuGet/NuGet.Client:backport/pr-6866-to-release/7.0.xCopy head branch name to clipboard