Put nixpkgs in NIX_PATH and system registry for flake configs

[antoineco]

Backport of NixOS/nixpkgs@e456032, which ensures that NIX_PATH includes nixpkgs=flake:nixpkgs when the system was built from a flake.

Here is the rationale for having NIX_PATH correctly set in a flake context, especially since Nix 2.24: NixOS/nixpkgs#273170 (comment)

---

💻 Demonstration from a REPL session, using my own system flake importing this branch

1. Before switch, despite having extra-nix-path = nixpkgs=flake:nixpkgs in nix.conf:

   $ nix repl --expr 'import <nixpkgs> {}'
   Nix 2.24.6
          error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

2. Open the REPL and load the flake's output to execute nix-darwin.lib.darwinSystem:

   $ nix repl
   Nix 2.24.6
   Type :? for help.
   >

   > darwinCfg = (builtins.getFlake (builtins.toString ./.)).darwinConfigurations.myhost

3. Verify that nix.registry was populated:

   • With nix.registry.nixpkgs equal to nix-darwin's nixpkgs.source (default)

     > darwinCfg.options.nix.registry.value.nixpkgs.to
     {
       path = { ... };
       type = "path";
     }
     > builtins.toPath darwinCfg.options.nix.registry.value.nixpkgs.to.path
     "/nix/store/1h99qq6970gkx3j0m9w4yrrl9y99y1nk-source"

   • With nix.registry.nixpkgs overriden by the user

     > darwinCfg.options.nix.registry.value.nixpkgs.to
     {
       type = "tarball";
       url = "https://flakehub.com/f/DeterminateSystems/nixpkgs-weekly/0.1.0.tar.gz";
     }

4. Verify that nix.nixPath was populated:

   • With nix.channel.enable = true (default)

     > darwinCfg.options.nix.nixPath.value
     [
       "nixpkgs=flake:nixpkgs"
       "/nix/var/nix/profiles/per-user/root/channels"
     ]

   • With nix.channel.enable = false

     > darwinCfg.options.nix.nixPath.value
     [ "nixpkgs=flake:nixpkgs" ]

5. After a switch, the system can perform <nixpkgs> lookups successfully:

   $ darwin-rebuild switch
   $ exec zsh -il
   $ nix repl --expr 'import <nixpkgs> {}'
   Nix 2.24.6
   Type :? for help.
   Loading installable ''...
   Added 22627 variables.

[emilazy]

I actually somewhat dislike this change because it makes nix flake lock behave weirdly for flakes that take a bare nixpkgs input (and IIRC I made the lock file of either nix-darwin or Home Manager weird for a while precisely because of this), but we should be consistent with NixOS here, so LGTM.