Ensure file-based apps' csc uses correct dotnet

[jjonescz]

Discovered while dogfooding a locally built SDK - when dotnet exec VBCSCompiler uses wrong dotnet.exe (e.g., the globally installed one instead of the locally built one), the compiler server launch fails (which means a 20 second timeout and fallback to MSBuild).

[baronfel]

This is good, but we probably need to do this in Program.cs as an explicit part of our startup, per some of the discussions we've been having with other teams. We already do this for MSBuild-based commands, it's time to expand the scope.

cc @dsplaisted @jaredpar

[RikkiGibson]

@baronfel so are you saying you are happy with this solution for now but want to change it in the future?

[baronfel]

yeah this is fine for now - but the whole CLI app needs to do this at a higher-level place, in part because of the whack-a-mole nature of doing it here and in umpteen other features.