add deadnix to formatter

flake-parts/formatting.nix

@@ -9,8 +9,21 @@
     treefmt.projectRootFile = "flake.nix";
     treefmt.flakeCheck = true;
     treefmt.flakeFormatter = true;
-    treefmt.programs.nixpkgs-fmt.enable = true;
     treefmt.programs.shellcheck.enable = true;
+    treefmt.settings.formatter.nix = {
+      command = "sh";
+      options = [
+        "-eucx"
+        ''
+          # First deadnix
+          ${lib.getExe pkgs.deadnix} --edit "$@"
+          # Then nixpkgs-fmt
+          ${lib.getExe pkgs.nixpkgs-fmt} "$@"
+        ''
+        "--" # this argument is ignored by bash
+      ];
+      includes = [ "*.nix" ];
+    };
     treefmt.settings.formatter.python = {
       command = "sh";
       options = [