[bst-1] Artifact proto timestamps are not updated on bst-artifact-server

The following discussion from !1540 (merged) should be addressed:

• @danielsilverstone-ct started a discussion: (+2 comments)

  Interestingly this never seems to update the mtime of the artifact proto itself.

Everything is in there, frankly. It looks like we forgot to expire artifact protos, and that we are leaking tiny bits of drive space whenever an artifact expires.