rnix-lsp: use nix (2.4)

[lovesegfault]

Motivation for this change

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

cc. ma27 to make sure this is OK

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.

[SuperSandro2000]

Does rnix-lsp exec nix somewhere and gets confused by warnings about new settings?

[Ma27]

We use nix __dump-builtins to provide builtins-completions, but this is optional (i.e. a hard-coded list of builtins is used if Nix 2.3 is in use, but no arg names or documentation is available in the completion then). In the end rnix-lsp should (and does) use the system-wide Nix.

We only need this for the tests to work fine.

[lovesegfault]

We use nix __dump-builtins to provide builtins-completions, but this is optional (i.e. a hard-coded list of builtins is used if Nix 2.3 is in use, but no arg names or documentation is available in the completion then). In the end rnix-lsp should (and does) use the system-wide Nix.

We only need this for the tests to work fine.

@Ma27 should I merge this?

[Ma27]

Yes, this doesn't have any implications on the runtime anyways.