chore: use nix fmt directly in ci

Author: yvan-sraka

.github/workflows/treefmt.yml

@@ -11,5 +11,5 @@ jobs:
             - uses: cachix/install-nix-action@v31
               with:
                   nix_path: nixpkgs=channel:nixos-unstable
-            - name: treefmt
-              run: nix-shell -p treefmt nixpkgs-fmt --run "treefmt --ci"
+            - name: nix fmt
+              run: nix fmt