emacs: use epg-gpg-program

Adjusted notmuch-crypto gpg call-process function to respect
the GPG program set by the EasyPG epg-gpg-program variable.

This is to correct a problem observed on NixOS where only gpg2 is
installed by default. The Notmuch Emacs frontend fails when trying to
access someone's key to verify their signature when it cannot find the
GPG binary.