Nix flake support

[GaetanLepage]

Feature description
Would it be interesting/chanllenging/hard for lorri to support nix flakes ?
By flake, I mean a flake having a devShell output section.
This is more a question of rather than a feature request.

Target users
More and more nix(os) users that are migrating to flakes.

[nyarly]

I've been using a shim shell.nix for flake work. It pretty good, but Lorri doesn't pick up the changes automatically whenflake.nix or flake.lock are changed.

It should be reasonably practical to get flakes supported directly, which is certainly a feature I'll want to look into supporting.

[haslersn]

Direnv now has an option use flake. Can lorri use this, or is that not helpful?

[nyarly]

I'm actually halfway through a PR to do flakes. It's suffered from deprioritization, but I think I'm well on the way.

@haslersn - use flake is adequate for many purposes, but note that it does a full flake build synchronously, so it can be several seconds (or minutes!) between prompts. Lorri has to sit "below" direnv, so direnv's use flake doesn't work for us.