Suppress another warning on Xenial

I've one xenial machine where the cron script emails me this line every night:
```
AppStream cache update completed, but some metadata was ignored due to errors.
```

Start by locating the cron script that checks Debian/Ubuntu updates and trace how apt output is collected and reported. Reproduce the Xenial warning if possible, then verify that this specific AppStream message is suppressed without hiding other update errors.

Written by the indexing model from the issue text.