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.

added 4 commits

• 0d17a9be...0661ade0 - 3 commits from branch master
• aeb50d9d - rebuilderd_worker: start removing stale chroots

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added 1 commit

• d1cabdfa - rebuilderd_worker: start removing stale chroots

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mentioned in commit 9512d912

merged