Add nix fmt hook

fixes: cachix#287

modules/hooks.nix

@@ -49,7 +49,7 @@ in
     ++ [ (mkRenamedOptionModule [ "settings" "rome" ] [ "hooks" "biome" "settings" ]) ]
     # Rename the remaining `settings.<name>` to `hooks.<name>.settings`
     ++ map (name: mkRenamedOptionModule [ "settings" name ] [ "hooks" name "settings" ])
-      [ "ansible-lint" "autoflake" "biome" "clippy" "cmake-format" "credo" "deadnix" "denofmt" "denolint" "dune-fmt" "eslint" "flake8" "headache" "hlint" "hpack" "isort" "latexindent" "lychee" "mkdocs-linkcheck" "mypy" "nixfmt" "ormolu" "php-cs-fixer" "phpcbf" "phpcs" "phpstan" "prettier" "psalm" "pylint" "pyright" "pyupgrade" "revive" "statix" ];
+      [ "ansible-lint" "autoflake" "biome" "clippy" "cmake-format" "credo" "deadnix" "denofmt" "denolint" "dune-fmt" "eslint" "flake8" "headache" "hlint" "hpack" "isort" "latexindent" "lychee" "mkdocs-linkcheck" "mypy" "nix" "nixfmt" "ormolu" "php-cs-fixer" "phpcbf" "phpcs" "phpstan" "prettier" "psalm" "pylint" "pyright" "pyupgrade" "revive" "statix" ];
 
   options.hookModule = lib.mkOption {
     type = types.deferredModule;
@@ -2894,6 +2894,13 @@ lib.escapeShellArgs (lib.concatMap (ext: [ "--ghc-opt" "-X${ext}" ]) hooks.ormol
             builtins.toString script;
           files = "\\.nix$";
         };
+      nix-fmt =
+        {
+          name = "nix-fmt";
+          description = "Nix formatter.";
+          package = tools.nix;
+          entry = "${hooks.nix.package}/bin/nix fmt";
+        };
       # nixfmt was renamed to nixfmt-classic.
       # The hook has been deprecated to free up the name for when the new RFC-style nixfmt becomes stable.
       nixfmt = nixfmt-classic;

nix/tools.nix

@@ -134,6 +134,7 @@ in
     mdl
     mdsh
     nil
+    nix
     nixpkgs-fmt
     ormolu
     pre-commit-hook-ensure-sops