Support nix 2.25 in formatter

Nix 2.25 changes the args passed to the formatter, so we now need to
handle an empty args.

See https://github.com/NixOS/nix/pull/11438

builtinModules/formatter.nix

@@ -42,6 +42,7 @@ in
           in
           pkgs.writeShellScriptBin "formatter" ''
             PATH=${if fullContext then "" else makeBinPath packages}
+            if [ $# -eq 0 ]; then exec "$0" .; fi
             for f in "$@"; do
               if [ -d "$f" ]; then
                 ${fd}/bin/fd "$f" -Htf -x "$0" &