mirror-to-mirror.sh: set 1h timeout for rsync runs

This script is run every 10 minutes or so to push new files to the primary mirrors. It sets a lock to prevent multiple copies from running in parallel. Yesterday, one rsync process got stuck and never exited, thereby preventing this script from running. Adding a 1h timeout seems like a safe way to deal with this kind of problem. This would not deal with rsync getting so stuck that it fails to heed the timeout, hopefully that's not an issue.

This version is already live and has been working.

FYI @licaon-kter @jspricke @grote @TheLastProject @FestplattenSchnitzel @linsui