don't unconditionally mirror in .gitlab-ci.yml

A few of us have experimented with using the .gitlab-ci.yml to run continuous integration jobs on gitlab.com and oniongit.eu. Right now they're failing in the "update" job because $DEPLOY_KEY isn't configured. At the very least we should make the job cleanly exit if the key is not configured.

Typical output from the "update" job:

$ eval $(ssh-agent -s)
Agent pid 4533
$ ssh-add <(echo "$DEPLOY_KEY")
Enter passphrase for /dev/fd/63: ERROR: Job failed: exit code 1