Enable build-via-msbuild by default for Windows.
Enable build-via-msbuild by default for Windows.

Any suggestions on how to better figure out where MSBuild lives are welcome.

A-Manning/FStar,hoheinzollern/FStar,MallabaevAzamat/FStar,A-Manning/FStar,YaccConstructor/FStar,hoheinzollern/FStar,MallabaevAzamat/FStar,hoheinzollern/FStar,YaccConstructor/FStar,hoheinzollern/FStar,YaccConstructor/FStar,A-Manning/FStar,YaccConstructor/FStar,MallabaevAzamat/FStar,MallabaevAzamat/FStar,FStarLang/FStar,FStarLang/FStar,FStarLang/FStar