Check for references to non-C: drives in the GCC search path

ghc#18774 (closed) points out that recent msys2 gcc builds (namely 10.1.0 used by https://gitlab.haskell.org/ghc/win32-tarballs/-/tags/0.2) include D: paths in the search path. This breaks on machines with optical storage devices without media.

We need to ask msys2 upstream to correct their build process and add a check in our CI infrastructure to ensure that the issue doesn't sneak in again.