In https://github.com/NixOS/nixpkgs/pull/120622 cleanup options were
added, but `remove-all-inc-of-but-n-full` was misspelled and as such
was not functioning.
(cherry picked from commit 0a977cf125a86b5580de6e05bfeaa07aa54c4a78)
Current module add backups forever, with no way to prune old ones.
Add an option to remove backups after n full backups or after some
amount of time.
Also run duplicity cleanup to clean unused files in case some previous
backup was improperly interrupted.
