remove unused archive index files from disk

[syphar]

Currently we store the archive index files locally after the build, and we never delete them.

We could track last-accessed for these, and remove index files that are not used for some time (a week?). Any request to a release that doesn't have a local index, will download it first.

We might be able to leverage some SQLite specifics, when the old cbor indexes are migrated.