nix-direnv: use nix (2.4) and remove enableFlakes

[lovesegfault]

Follow-up to #144197 since nixUnstable is no longer needed for flake
support.

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[roberth]

Maybe nix shouldn't be baked into this package at all; couldn't it just use the environment's Nix? That would make it easier for users to upgrade their nix to whichever version works best for them, considering that flakes are still in development.

[lovesegfault]

Maybe nix shouldn't be baked into this package at all; couldn't it just use the environment's Nix? That would make it easier for users to upgrade their nix to whichever version works best for them, considering that flakes are still in development.

Maybe? @Mic92 probably knows best here.

[Mic92]

Maybe nix shouldn't be baked into this package at all; couldn't it just use the environment's Nix? That would make it easier for users to upgrade their nix to whichever version works best for them, considering that flakes are still in development.

If someone does not want to use flakes, they don't have to. use flake is independent from use nix.

[roberth]

Maybe nix shouldn't be baked into this package at all; couldn't it just use the environment's Nix? That would make it easier for users to upgrade their nix to whichever version works best for them, considering that flakes are still in development.

If someone does not want to use flakes, they don't have to. use flake is independent from use nix.

True, but not what I was trying to say. I think some people will want to upgrade from stable nix (which also supports flakes) to nixUnstable which will contain more fixes.
With the current setup, this requires users to set nix in an overlay, which seems unnecessary.
Most tools don't pin Nix, so I wonder if this one needs to.

[Mic92]

Maybe nix shouldn't be baked into this package at all; couldn't it just use the environment's Nix? That would make it easier for users to upgrade their nix to whichever version works best for them, considering that flakes are still in development.

If someone does not want to use flakes, they don't have to. use flake is independent from use nix.

True, but not what I was trying to say. I think some people will want to upgrade from stable nix (which also supports flakes) to nixUnstable which will contain more fixes. With the current setup, this requires users to set nix in an overlay, which seems unnecessary. Most tools don't pin Nix, so I wonder if this one needs to.

I personally don't care if it is pinned or not. I introduced this hard-coding based on feedback with other people: nix-community/nix-direnv#52 cc @anka-213 @hmenke

[roberth]

I personally don't care if it is pinned or not.

I do care for the poor soul who upgrades their Nix and stuff still doesn't work. Sadly, I don't have infinite time.