reproducible: move the db_backup task to its own job

this task takes quite a long time even on an idle host, mostly in the
compression (`xz`) stage.  We are talking about hours just doing that
when the host is busy.

That precludes and delays the rest of the maintenance tasks, that are
often more relevant and somewhat time-sensitive (like updating the
apt-venv lists).

Signed-off-by: Mattia Rizzolo <mattia@debian.org>