Improve workflows

[KevinRansom]

Improve some of the build workflows in the new build.

1. Adds ngen to build for desktop and ngen builds of fsi.exe and fsc.exe, when run with administrator privileges
2. Correct the check for existence of proto compiler before rebuilding it. (Was rebuilding always)
3. Build proto. Once again only rebuilds the proto compiler not the coreclr and desktop compilers
4. Ensure that Proto build only builds the desktop version of the proto compiler.