nixos/nix-direnv: respect configured nix package

[Mic92]

Rebuilds should be reasonably fast since it's a shell script and we don't end up with different versions of nix in the store.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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 rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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.

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[r-vdp]

Should we not instead make this the default of the nix-direnv.package option, so that it can still be overridden if needed?

[szlend]

Should we not instead make this the default of the nix-direnv.package option, so that it can still be overridden if needed?

That's a good point, though then you need to remember to override nix if you for example take nix-direnv from unstable. But if nix-community/nix-direnv#451 is resolved and we prefer to pick up nix from PATH, then this is not a big issue I suppose.

[donovanglover]

Ran into this issue today since NixOS/nix#5759 (fixed in NixOS/nix#6614) is in Nix 2.19 but not 2.18.