perf: import treefmt-nix via flake-parts

flake.lock

@@ -461,22 +461,6 @@
         "type": "github"
       }
     },
-    "nixpkgs_4": {
-      "locked": {
-        "lastModified": 1735554305,
-        "narHash": "sha256-zExSA1i/b+1NMRhGGLtNfFGXgLtgo+dcuzHzaWA6w3Q=",
-        "owner": "nixos",
-        "repo": "nixpkgs",
-        "rev": "0e82ab234249d8eee3e8c91437802b32c74bb3fd",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nixos",
-        "ref": "nixpkgs-unstable",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "nuscht-search": {
       "inputs": {
         "flake-utils": "flake-utils",
@@ -636,7 +620,9 @@
     },
     "treefmt-nix": {
       "inputs": {
-        "nixpkgs": "nixpkgs_4"
+        "nixpkgs": [
+          "nixpkgs"
+        ]
       },
       "locked": {
         "lastModified": 1736154270,

flake.nix

@@ -7,6 +7,7 @@
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
     treefmt-nix.url = "github:numtide/treefmt-nix";
+    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
     impermanence.url = "github:nix-community/impermanence/master";
     home-manager.url = "github:nix-community/home-manager";
     home-manager.inputs.nixpkgs.follows = "nixpkgs";
@@ -24,18 +25,16 @@
   outputs = inputs@{self, flake-parts, treefmt-nix,...}:
   flake-parts.lib.mkFlake
   { inherit inputs; } {
-    perSystem = {pkgs, ...}: let
-      treefmtEval = treefmt-nix.lib.evalModule pkgs {
-        projectRootFile = "flake.nix";
-        programs.nixfmt.enable = true;
-      };
-    in {
-      formatter = treefmtEval.config.build.wrapper;
-      checks.formatting = treefmtEval.config.build.check self;
-    };
     systems = [
       "x86_64-linux"
     ];
-    imports = [ ./spec ];
+
+    perSystem.treefmt = {
+      programs.nixfmt.enable = true;
+    };
+    imports = [
+      ./spec
+      treefmt-nix.flakeModule
+    ];
   };
 }