Implement optional backup for workers

CI worker environments get rebuilt daily (debci update-worker). This makes reproducing certain errors, like this one, difficult.

On hosts that can afford the disk space, an option to store compressed versions of the images for a certain period could be useful. This could be staggered, e.g.: daily for the first two weeks, then weekly, then monthly.

On my end, an xz-compressed version of unstable.img used about 227MB disk space.