stylix: adopt flake-utils.lib.eachDefaultSystem

Adopt the flake-utils.lib.eachDefaultSystem function for added features
like '--impure' flag support [1].

[1]: https://github.com/numtide/flake-utils/pull/115

Link: https://github.com/danth/stylix/pull/515

flake.nix

@@ -65,13 +65,12 @@
 
   outputs =
     { nixpkgs, base16, self, ... }@inputs:
-    {
-      packages = nixpkgs.lib.genAttrs inputs.flake-utils.lib.defaultSystems (
-        system:
-        let
+    inputs.flake-utils.lib.eachDefaultSystem (
+      system: let
         inherit (nixpkgs) lib;
         pkgs = nixpkgs.legacyPackages.${system};
-
+      in {
+        packages = let
             universalPackages = {
               docs = import ./docs { inherit pkgs inputs lib; };
               palette-generator = pkgs.callPackage ./palette-generator { };
@@ -83,9 +82,10 @@
               (lib.hasSuffix "-linux" system)
               (import ./stylix/testbed.nix { inherit pkgs inputs lib; });
           in
-          universalPackages // testbedPackages
-      );
-
+            universalPackages // testbedPackages;
+      }
+    )
+    // {
       nixosModules.stylix = { pkgs, ... }@args: {
         imports = [
           (import ./stylix/nixos inputs {