Add nix fmt hook

fixes: cachix#287

Author: shikanime

modules/hooks.nix

@@ -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 = "Format Nix files using the formatter specified in your flake.";
+          package = tools.nix;
+          entry = "${hooks.nix-fmt.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

@@ -44,6 +44,7 @@
 , mdl
 , mdsh
 , nil
+, nix
 , nixfmt
 , nixfmt-classic ? null
 , nixfmt-rfc-style ? null
@@ -134,6 +135,7 @@ in
     mdl
     mdsh
     nil
+    nix
     nixpkgs-fmt
     ormolu
     pre-commit-hook-ensure-sops