Fix Debian copyright mirror missing yaml

Our Debian copyright mirror job has not been running because the yaml package it was depending on seems to now be unavailable to its python interpreter.

#5638 tried to fix it, but it seems to still be happening.

Start with the Debian copyright mirror job and inspect the yaml dependency in the Python interpreter environment. Review issue #5638 to understand the attempted fix and verify the job can run successfully with the available package.

Written by the indexing model from the issue text.