Periodic cleanup of cached update artifacts

[iliana]

Nexus stores update artifacts in a temporary path which will be wiped on boot(? maybe??) — but we should clean up older files from the cache, and maybe even remove least-recently-used files if we're running low on space.

(From #717)

[sunshowers]

This will be required at some point with the new design in RFD 424, but is not required for MVP (we can just live with ~6 months of not-cleaned-up update artifacts).

[davepacheco]

@iliana Is this resolved by the recent repo depot work?