lenovo-legion: use unstable update script; fix version

At the time of writing this, the `0.0.21` tag does not exist, upstream.

Closes: https://github.com/NixOS/nixpkgs/pull/450601

pkgs/os-specific/linux/lenovo-legion/app.nix

@@ -4,11 +4,12 @@
   xorg,
   wrapQtAppsHook,
   python3,
+  nix-update-script,
 }:
 
 python3.pkgs.buildPythonApplication rec {
   pname = "lenovo-legion-app";
-  version = "0.0.21-unstable-2025-07-11";
+  version = "0.0.20-unstable-2025-07-11";
   format = "setuptools";
 
   src = fetchFromGitHub {
@@ -52,6 +53,8 @@ python3.pkgs.buildPythonApplication rec {
     makeWrapperArgs+=("''${qtWrapperArgs[@]}")
   '';
 
+  passthru.updateScript = nix-update-script { extraArgs = [ "--version=branch" ]; };
+
   meta = {
     description = "Utility to control Lenovo Legion laptop";
     homepage = "https://github.com/johnfanv2/LenovoLegionLinux";