sign in the builddir where the files actually are

Without this, I get this output:
....
mediawiki-i18n-1.22.0rc3.patch.gz written

You need a passphrase to unlock the secret key for
user: "Mark A. Hershberger <mah@everybody.org>"
2048-bit RSA key, ID 7F901A30, created 2009-07-01

gpg: can't open `CURRENT-DIR/mediawiki-core-1.22.0rc3.tar.gz': No such file or directory
gpg: signing failed: file open error
gpg failed, exiting
$

Change-Id: Ibc927cce36b0ce8c3cd45b44ed33602b72d3132d