maintainers/scripts/update: Mark queue jobs as done

I believe this is only relevant if we were to `join` the queue itself but it is a good practice anyway.
https://docs.python.org/3/library/asyncio-queue.html#asyncio.Queue.task_done

maintainers/scripts/update.py

@@ -242,6 +242,8 @@ async def updater(
 
         await run_update_script(nixpkgs_root, merge_lock, temp_dir, package, keep_going)
 
+        packages_to_update.task_done()
+
 
 async def start_updates(
     max_workers: int,