rebuilderd_worker: clean the package cache as well

Using paccache for this which keeps up to 3 versions per package, and
also instructing it to keep any packages accessed in the past 2 weeks.

roles/rebuilderd_worker/files/clean-repro

 #!/bin/bash -e
 
+# remove leftover chroots that are more than a week old
 find /var/lib/repro -maxdepth 1 -name '*?_?*' -mtime +6 -exec rm -rf {} +
+
+# clean the package cache but keep recently accessed files
+flock /var/lib/rebuilderd-worker/cache.lock \
+  paccache -r -q -c /var/lib/rebuilderd-worker/cache --min-atime '2 weeks ago'

roles/rebuilderd_worker/files/clean-repro.service

 [Unit]
-Description=Clean up stale repro chroots
+Description=Clean up rebuilderd-worker chroots and cache
 ConditionPathExists=/var/lib/repro
+ConditionPathExists=/var/lib/rebuilderd-worker/cache
 
 [Service]
 Type=oneshot

roles/rebuilderd_worker/files/clean-repro.timer

 [Unit]
-Description=Daily repro cleanup
+Description=Daily rebuilderd-worker chroot and cache cleanup
 
 [Timer]
 OnCalendar=daily