tests/azure: Temporarily stay at Ubuntu 20.04

Github has been migrating ubuntu-latest from 20.04 to 22.04. 22.04 comes
with cgroups version 2.

No tests are run at the momens as the setup of the test container always
fails with "Failed to create temporary directory" for gathering facts.

See also:

https://github.com/ansible-collections/news-for-maintainers/issues/28