Feature request: Option to keep X latest direnv gcroots

[sportshead]

Currently if I run nh clean all --keep-since 7d I get to keep all the direnv gcroots that are at most 7 days old. However there are some projects that I haven't used in 7d, and I would like to keep at least one gcroot for each of them, so that I don't have to rebuild the devshell when I go back to them.

Maybe the --keep X could also be used for direnv gcroots? Or a separate option could be used to manage direnv gcroots instead.

[NotAShelf]

This can be implemented, but I'd like to separate it from the --keep flag.

That said, I currently lack the bandwidth to deal with this. It's obviously trivial, but there are a few other PRs that I'd like to finish before I get distracted again. Not to mention other projects that I'm hoping to finish.

If anyone would like to submit a PR, that'd be great. Otherwise this'll remain on the backlog until I have some more free time or a need for this flag personally.