maintainers/scripts/update.nix: Ensure the worktree is clean

When an update script fails, it might still modify the source tree.
These changes would then be committed in the next update attempt.
Let’s make sure the worktree is clean before updating to avoid that.

maintainers/scripts/update.py

@@ -39,6 +39,9 @@ async def run_update_script(nixpkgs_root: str, merge_lock: asyncio.Lock, temp_di
     if temp_dir is not None:
         worktree, _branch = temp_dir
 
+        # Ensure the worktree is clean before update.
+        await check_subprocess('git', 'reset', '--hard', '--quiet', 'HEAD', cwd=worktree)
+
         # Update scripts can use $(dirname $0) to get their location but we want to run
         # their clones in the git worktree, not in the main nixpkgs repo.
         update_script_command = map(lambda arg: re.sub(r'^{0}'.format(re.escape(nixpkgs_root)), worktree, arg), update_script_command)