chore(nix): pin nixpkgs via flake.lock for reproducible runs

https://chatgpt.com/share/694ab489-7028-800f-8398-a19a99faffd0

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1766309749,
+        "narHash": "sha256-3xY8CZ4rSnQ0NqGhMKAy5vgC+2IVK0NoVEzDoOh4DA4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "a6531044f6d0bef691ea18d4d4ce44d0daa6e816",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}