nixos/nix-channel: don't set nix-path (#327683)

Otherwise, the empty path in `nix.conf` takes precedence over `NIX_PATH`,
and by extension the `nix.nixPath` configuration option.

Introduced in 61afc4d166.

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -291,6 +291,9 @@
   {option}`services.gitlab-runner.services.<name>.authenticationTokenConfigFile` instead of the former
   {option}`services.gitlab-runner.services.<name>.registrationConfigFile` option.
 
+- `nix.channel.enable = false` no longer implies `nix.settings.nix-path = []`.
+  Since Nix 2.13, a `nix-path` set in `nix.conf` cannot be overriden by the `NIX_PATH` configuration variable.
+
 ## Detailed migration information {#sec-release-24.11-migration}
 
 ### `sound` options removal {#sec-release-24.11-migration-sound}

nixos/modules/config/nix-channel.nix

@@ -94,8 +94,6 @@ in
       NIX_PATH = cfg.nixPath;
     };
 
-    nix.settings.nix-path = mkIf (! cfg.channel.enable) (mkDefault "");
-
     systemd.tmpfiles.rules = lib.mkIf cfg.channel.enable [
       ''f /root/.nix-channels - - - - ${config.system.defaultChannel} nixos\n''
     ];