pre-commit: add 'nixpkgs-fmt' hook

.pre-commit-config.yaml

@@ -10,3 +10,12 @@ repos:
   rev: '2.1.4'
   hooks:
   - id: 'forbid-binary'
+- repo: 'local'
+  hooks:
+  - id: 'nixpkgs-fmt'
+    name: 'nixpkgs-fmt'
+    description: 'Format nix code with nixpkgs-fmt'
+    entry: 'nixpkgs-fmt'
+    language: 'system'
+    files: '\.nix$'
+    always_run: true