Fix streaming zip files from canonical datasets
Previous PR #2798 fixed streaming remote zip files when passing the parameter `data_files`.

However, that broke streaming zip files used in canonical `datasets` scripts, which normally have a subsequent `join()` (patched with `xjoin()`) after the `StreamingDownloadManager.download_and_extract()` is called.

This PR fixes this issue and allows streaming zip files both from:
- canonical datasets scripts and
- data files.