ngclient: Advisory locking for artifacts

This should prevent issues with multiple processes trying to write at
same time.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

tuf/ngclient/updater.py

@@ -337,6 +337,7 @@ def download_target(
 
             target_file.seek(0)
             with open(filepath, "wb") as destination_file:
+                _lock_file(destination_file)
                 shutil.copyfileobj(target_file, destination_file)
 
         logger.debug("Downloaded target %s", targetinfo.path)