archivetools will add missing packages to the "packages" directory in
the archive[1], but it is only done once a day, so if a package is
updated more than once in that timeframe, the earlier version will not
be added to the archive.

For this reason, we need to sync archived packages from repos.a.o to
gemini.a.o, so we don't lose packages, but also because we want the
archived packages to show up faster on the archive server than once a
day.

The package files could already exist on gemini, created by
archivetools, in which case rsync just ignores the files
(--ignore-existing).

The archived package files are deleted on repos.a.o after three days, so
there is some leeway to stop archive-cleanup.timer if the sync breaks
for whatever reason.

[1] https://gitlab.archlinux.org/archlinux/archivetools/-/blob/d5cbd5a5a9454eecd5c46b64ebc08d5087a25f2d/archive.sh#L141-L200

Relates to #531