flake: format Nix with nixfmt

flake-checks moved its treefmt Nix formatter to nixfmt, so every .nix file
failed the formatting check. The devShell and the pre-commit hook shipped
nixpkgs-fmt, which reformatted them straight back.

flake.lock

@@ -9,11 +9,11 @@
         "treefmt-nix": "treefmt-nix"
       },
       "locked": {
-        "lastModified": 1783015821,
-        "narHash": "sha256-vmzYTZxIAy3OYXwkRL5GQYk8fNnh8TLWcxQRIO3ywOU=",
+        "lastModified": 1788072657,
+        "narHash": "sha256-KJ62kJIcGVq3bxaXrKLdHpxyzsb2HYnx99/jqlxMYIw=",
         "owner": "kradalby",
         "repo": "flake-checks",
-        "rev": "3c821706eb0bd07f515f9a7726b650ccae927433",
+        "rev": "13e811640f0cdec4974062d0d43208cec5bdad6c",
         "type": "github"
       },
       "original": {