Opt into prerel SDKs when in VS/MSBuild.exe-driven builds
Opt into prerel SDKs when in VS/MSBuild.exe-driven builds

This should fix CI builds running on older VS images.

AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild