flake: pin Nixpkgs explicitly

flake.lock

@@ -2,16 +2,18 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1718149104,
-        "narHash": "sha256-Ds1QpobBX2yoUDx9ZruqVGJ/uQPgcXoYuobBguyKEh8=",
+        "lastModified": 1736241350,
+        "narHash": "sha256-CHd7yhaDigUuJyDeX0SADbTM9FXfiWaeNyY34FL1wQU=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "e913ae340076bbb73d9f4d3d065c2bca7caafb16",
+        "rev": "8c9fd3e564728e90829ee7dbac6edc972971cd0f",
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
-        "type": "indirect"
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "root": {

flake.nix

@@ -1,6 +1,10 @@
 {
   description = "A collection of darwin modules";
 
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
   outputs = { self, nixpkgs }: let
     forAllSystems = nixpkgs.lib.genAttrs [ "aarch64-darwin" "x86_64-darwin" "aarch64-linux" "x86_64-linux" ];
     forDarwinSystems = nixpkgs.lib.genAttrs [ "aarch64-darwin" "x86_64-darwin" ];