upstream CI: Enable nightly tests using ansible-core 2.12.

This patch modifies the Python version used to be the latest available,
and add stages to execute the tests using ansible-core 2.12. As we
use Ubuntu 20.04, Python version 3.8 is avaiable.

Previously, ansible-core 2.12 was not available as it cannot be
installed with Python 3.6, which was the version used.