Look for GnuPG executables in the GnuPG install path

If, for some reason, we don't get the path of a GnuPG executable from
its engine info then look for the executable in the GnuPG install path
instead of in PATH. This ensures that we always use the same executables
as gpgme.

(cherry picked from commit bfec230af060b9a77084f1a05db9151b9b506a4a)

• GnuPG-bug-id: T7971