We found that new data (since we removed -t) was no correctly being
skipped for re-download. We have found that this doesn't happen with
-t on later rsyncs, which have included fixes for -t to not touch the
timestamps if things are not updated. We have updated mirror-update
to Focal that has this rsync, so restore the flag.
Change-Id: I3fa16dbf6487a442549c540796807ef4916d4e6e
a58ad34a5c