nixos/nix: move nix.nixPath into nix.settings.nix-path

This removes some confusion when setting nix.settings.nix-path to a custom value,
disabling channels and ending up with an empty NIX_PATH.
This is important since nix 2.24 fixed NIX_PATH not overwriting the
nix-path setting.

nixos/modules/config/nix-channel.nix

@@ -42,7 +42,7 @@ in
         };
       };
 
-      nixPath = mkOption {
+      settings.nix-path = mkOption {
         type = types.listOf types.str;
         default =
           if cfg.channel.enable then
@@ -80,8 +80,11 @@ in
     };
   };
 
-  config = mkIf cfg.enable {
+  imports = [
+    (lib.mkRenamedOptionModule [ "nix" "nixPath" ] [ "nix" "settings" "nix-path" ])
+  ];
 
+  config = mkIf cfg.enable {
     environment.extraInit = mkIf cfg.channel.enable ''
       if [ -e "$HOME/.nix-defexpr/channels" ]; then
         export NIX_PATH="$HOME/.nix-defexpr/channels''${NIX_PATH:+:$NIX_PATH}"
@@ -95,7 +98,7 @@ in
     # NIX_PATH has a non-empty default according to Nix docs, so we don't unset
     # it when empty.
     environment.sessionVariables = {
-      NIX_PATH = cfg.nixPath;
+      NIX_PATH = cfg.settings.nix-path;
     };
 
     systemd.tmpfiles.rules = lib.mkIf cfg.channel.enable [

nixos/modules/misc/nixpkgs-flake.nix

@@ -102,7 +102,7 @@ in
         # because we would need some kind of evil shim taking the *calling* flake's self path,
         # perhaps, to ever make that work (in order to know where the Nix expr for the system came
         # from and how to call it).
-        nix.nixPath = lib.mkDefault (
+        nix.settings.nix-path = lib.mkDefault (
           [ "nixpkgs=flake:nixpkgs" ]
           ++ lib.optional config.nix.channel.enable "/nix/var/nix/profiles/per-user/root/channels"
         );

nixos/tests/nix-required-mounts/default.nix

@@ -12,10 +12,12 @@ in
     {
       virtualisation.writableStore = true;
       system.extraDependencies = [ (pkgs.runCommand "deps" { } "mkdir $out").inputDerivation ];
-      nix.nixPath = [ "nixpkgs=${../../..}" ];
-      nix.settings.substituters = lib.mkForce [ ];
-      nix.settings.system-features = [ "supported-feature" ];
-      nix.settings.experimental-features = [ "nix-command" ];
+      nix.settings = {
+        experimental-features = [ "nix-command" ];
+        nix-path = [ "nixpkgs=${../../..}" ];
+        substituters = lib.mkForce [ ];
+        system-features = [ "supported-feature" ];
+      };
       nix.enable = true; # disabled by default. See all-tests.nix / tag(no-nix-by-default)
       programs.nix-required-mounts.enable = true;
       programs.nix-required-mounts.allowedPatterns.supported-feature = {