flake.nix: cleanup, refactor

flake.nix

@@ -3,7 +3,7 @@
 
   inputs = {
     # Main nixpkgs (used for gnused)
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    nixpkgs.url = "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz";
 
     # Pinned nixpkgs for kubernetes-controller-tools
     # Search: https://www.nixhub.io/packages/kubernetes-controller-tools
@@ -12,33 +12,34 @@
     # Pinned nixpkgs for golangci-lint
     # Search: https://www.nixhub.io/packages/golangci-lint
     nixpkgs-golangci.url = "github:NixOS/nixpkgs/80d901ec0377e19ac3f7bb8c035201e2e098cc97";
-
-    flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { self, nixpkgs, nixpkgs-kct, nixpkgs-golangci, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
-      let
-        pkgs = import nixpkgs {
-          inherit system;
-        };
+  outputs =
+    {
+      nixpkgs,
+      nixpkgs-kct,
+      nixpkgs-golangci,
+      ...
+    }:
+    let
+      inherit (nixpkgs.lib) genAttrs;
+      forEachSystem = genAttrs nixpkgs.lib.systems.flakeExposed;
 
-        pkgs-kct = import nixpkgs-kct {
-          inherit system;
-        };
-
-        pkgs-golangci = import nixpkgs-golangci {
-          inherit system;
-        };
-      in
-      {
-        devShells.default = pkgs.mkShell {
+      pkgsForEach = nixpkgs.legacyPackages;
+      pkgsKctForEach = nixpkgs-kct.legacyPackages;
+      pkgsGolangCiForEach = nixpkgs-golangci.legacyPackages;
+    in
+    {
+      devShells = forEachSystem (system: {
+        default = pkgsForEach.${system}.mkShell {
           packages = [
-            pkgs-kct.kubernetes-controller-tools
-            pkgs.gnused
-            pkgs-golangci.golangci-lint
+            pkgsForEach.${system}.gnused
+            pkgsKctForEach.${system}.kubernetes-controller-tools
+            pkgsGolangCiForEach.${system}.golangci-lint
           ];
         };
-      }
-    );
-}
+      });
+
+      formatter = forEachSystem (system: pkgsForEach.${system}.nixfmt);
+    };
+}