flake.nix: use flake-utils for system gen

flake.nix

@@ -3,33 +3,37 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
   };
 
   outputs =
-    { self, nixpkgs }:
-    let
-      supportedSystems = [
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachSystem
+      [
         "x86_64-linux"
         "aarch64-linux"
-      ];
-      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
-    in
-    {
-      nixosModules.default = import ./module.nix;
-      nixosModules.arr-init = import ./module.nix;
-
-      checks = forAllSystems (
+      ]
+      (
         system:
         let
           pkgs = nixpkgs.legacyPackages.${system};
         in
-        import ./tests {
-          inherit pkgs;
-          lib = nixpkgs.lib;
-          inherit self;
-        }
-      );
+        {
+          checks = import ./tests {
+            inherit pkgs;
+            lib = nixpkgs.lib;
+            inherit self;
+          };
 
-      formatter = forAllSystems (system: nixpkgs.legacyPackages.${system}.nixfmt-tree);
+          formatter = pkgs.nixfmt-tree;
+        }
+      )
+    // {
+      nixosModules.default = import ./module.nix;
+      nixosModules.arr-init = import ./module.nix;
     };
 }