grovel: fix %INVOKE on Windows
grovel: fix %INVOKE on Windows

Instead of redirecting the output, we just open a stream to the
process and read from the stream directly instead, thus achieving the
desired result. This should also work with any platform in the future
that has a problem with output redirection.

mcna/cffi,attila-lendvai/cffi,stassats/cffi,3b/cffi,attila-lendvai/cffi,mcna/cffi,dkochmanski/cffi,dkochmanski/cffi,dkochmanski/cffi,3b/cffi,stassats/cffi,cffi/cffi,cffi/cffi,stassats/cffi,3b/cffi,cffi/cffi