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.

roles/rebuilderd_worker/files/clean-repro

+#!/bin/bash -e
+
+find /var/lib/repro -maxdepth 1 -name '*?_?*' -mtime +6 -exec rm -rf {} +