Use ComSpec when GetPathToSystemFile(cmd) is bogus
Use ComSpec when GetPathToSystemFile(cmd) is bogus

Fixes #2273 by falling back to `%ComSpec%` when
`ToolLocationHelper.GetPathToSystemFile("cmd.exe")` returns a
nonexistent value, as it does when `SHGetFolderPath` returns the user's
home directory instead of the system directory, which sometimes happens
on Nano Server.

jeffkl/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,cdmihai/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,mono/msbuild,cdmihai/msbuild,rainersigwald/msbuild,jeffkl/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,Microsoft/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,mono/msbuild,chipitsine/msbuild,mono/msbuild,Microsoft/msbuild,Microsoft/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,chipitsine/msbuild