virttest.build_helper: Change temporary GNUPG home dir

It is better to create the temporary GNUPG home dir in
the data_dir.TMP_DIR rather than in the same directory
as the key file, to avoid reuse from different jobs
and also, the gpg mount might be read-only.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>