Log Message:
notmuch: Reuse GPGME_GPG variable rather than inventing our own.

Not only cleaner, but also fixes a build issue seen on macOS likely related to
variable definition ordering, where GPG was not set correctly and ended up
trying to use a non-existent "gpg" command.  This change has the added \ 
benefit
of using the full path to the gpg binary instead of relying on PATH.