Nix 2.4 release notes

[edolstra]

Fixes #4521.

[abathur]

I don't know if the release note gets circulated widely enough that it's important to nail the descriptions of nix develop, nix shell, and nix run, but this discussion on discourse (from the linked post to the end) has some good context on how people are making sense of these: https://discourse.nixos.org/t/difference-between-nix-shell-and-nix-run/2951/14

[Ericson2314]

Not sure if this means we're about to cut a branch, but in case it does I started cleaning up some things:

• Test nix build --json return output paths in floating CA case #4589 is now just testing, always good to test more things!
• std::visit by reference #5311 might help with performance, ensuring std::visit doesn't result in extra copying, which we didn't have in Nix 2.3 any any event.

[edolstra]

@Ericson2314 Do you mean you want those to be merged before or after branching?

[Ericson2314]

@edolstra Before.