chore(nix): use nixfmt instead of nixpkgs-fmt

- It won't complain if there is no nix on your machine
  (but then why would you touch nix files?)
- It'll run only for changed files

The code style is changed to match with RFC-166[^1]

[^1]: NixOS/rfcs#166

.github/workflows/run-tests.yml

@@ -23,6 +23,11 @@ jobs:
       - name: Setup 3rd party dependencies
         uses: ./.github/actions/setup-dependencies
 
+      - name: Setup Nix
+        uses: cachix/install-nix-action@v30
+        with:
+          install_url: https://releases.nixos.org/nix/nix-2.18.8/install
+
       - name: Check pre-commit hook
         uses: pre-commit/action@v3.0.1
 

.pre-commit-config.yaml

@@ -29,18 +29,24 @@ repos:
         types_or: [python, pyi]
         require_serial: true
         entry: poetry run ruff format --force-exclude
+      - id: nixfmt
+        name: Format Nix (nixfmt)
+        types: [nix]
+        language: system
+        require_serial: true
+        entry: bash -c 'command -v nix && nix "$@" || echo "skipping nixfmt"' nix
+        args:
+          - --experimental-features
+          - nix-command flakes
+          - --accept-flake-config
+          - fmt
 
   - repo: https://github.com/jendrikseipp/vulture
     rev: v2.7
     hooks:
       - id: vulture
         name: Check vulture
 
-  - repo: https://github.com/nix-community/nixpkgs-fmt
-    rev: v1.3.0
-    hooks:
-      - id: nixpkgs-fmt
-
   - repo: https://github.com/python-jsonschema/check-jsonschema
     rev: 0.21.0
     hooks:

flake.nix

@@ -83,5 +83,7 @@
         });
 
       legacyPackages = forAllSystems (system: nixpkgsFor.${system});
+
+      formatter = forAllSystems (system: nixpkgsFor.${system}.nixfmt-rfc-style);
     };
 }