Make gpg-zip use tar from $PATH

Apparently there is no clean way to configure this from ./configure,
and upstream is deprecating gpg-zip anyway. So just force-set tar to
be manually "tar" (meaning, that we should look in the $PATH at
runtime).

See also https://dev.gnupg.org/T4251 and https://bugs.debian.org/913582

Gbp-Pq: Name Make-gpg-zip-use-tar-from-PATH.patch