stylix: re-add flake-utils dependency and interface flake systems

Re-add the flake-utils dependency removed in commit ff5da2914c
("Remove dependency on flake-utils ➖") and interface
flake systems using the "externally extensible flake systems" [1]
pattern.

[1]: https://github.com/nix-systems/nix-systems

Closes: https://github.com/danth/stylix/issues/512
Link: https://github.com/danth/stylix/pull/515

flake.lock

@@ -130,6 +130,26 @@
         "type": "github"
       }
     },
+    "flake-utils": {
+      "inputs": {
+        "systems": [
+          "systems"
+        ]
+      },
+      "locked": {
+        "lastModified": 1710146030,
+        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "fromYaml": {
       "flake": false,
       "locked": {
@@ -209,9 +229,24 @@
         "base16-tmux": "base16-tmux",
         "base16-vim": "base16-vim",
         "flake-compat": "flake-compat",
+        "flake-utils": "flake-utils",
         "gnome-shell": "gnome-shell",
         "home-manager": "home-manager",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "systems": "systems"
+      }
+    },
+    "systems": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1,
+        "narHash": "sha256-4CDjAaoWvkUTFjjKdcDZG85k4vQfgR0K4+zgtqTW5ho=",
+        "path": "stylix/systems.nix",
+        "type": "path"
+      },
+      "original": {
+        "path": "stylix/systems.nix",
+        "type": "path"
       }
     }
   },

flake.nix

@@ -37,6 +37,11 @@
       url = "github:edolstra/flake-compat";
     };
 
+    flake-utils = {
+      inputs.systems.follows = "systems";
+      url = "github:numtide/flake-utils";
+    };
+
     gnome-shell = {
       flake = false;
 
@@ -53,18 +58,18 @@
     };
 
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+    # Interface flake systems.
+    systems = {
+      flake = false;
+      url = "path:stylix/systems.nix";
+    };
   };
 
   outputs =
     { nixpkgs, base16, self, ... }@inputs:
     {
-      packages = nixpkgs.lib.genAttrs [
+      packages = nixpkgs.lib.genAttrs inputs.flake-utils.lib.defaultSystems (
-        "aarch64-darwin"
-        "aarch64-linux"
-        "i686-linux"
-        "x86_64-darwin"
-        "x86_64-linux"
-      ] (
         system:
         let
           inherit (nixpkgs) lib;

stylix/systems.nix

@@ -0,0 +1,7 @@
+[
+  "aarch64-darwin"
+  "aarch64-linux"
+  "i686-linux"
+  "x86_64-darwin"
+  "x86_64-linux"
+]