rebuilderd_worker: start removing stale chroots

On some build failures, or perhaps when rebooting the boxes while a build is ongoing, repro chroot can be left behind, taking up space.

Add a service with a daily timer to remove week old chroots.

host_vars/mirror.pkgbuild.com/misc

 ---
 mirror_domain: mirror.pkgbuild.com
 mirror_debug_packages: false
+geomirror_acme_challenge: true
 archweb_mirrorcheck_locations: [20, 21]
 filesystem: btrfs