Add brioche clean subcommand

[kylewlacy]

Right now, running dust ~/.local/share/brioche shows Brioche using 58 GB for me locally! Some of that is stuff I'd want to keep, others are things like failed temp process dirs, others are builds I don't need any more. We should have a command like brioche clean to help remove un-needed stuff.

This sounds simple on the surface, but the hard part is deciding what stuff is un-needed or unwanted. Things like unused temp dirs are good candidates for deletion, but others will likely require more metadata that we aren't tracking today