gnupg: Be deterministic about sendmail

Set a path to where sendmail would exist making the output deterministic
as it no longer depends on the build host and the presense of sendmail
there.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 32e03a43)
Signed-off-by: Steve Sakoman <steve@sakoman.com>