Run Focal debs tests in CI

[emkll]

Description

We can now build SecureDrop deb packages for Focal thanks to the changes introduced in #5465. We already test the debs in Xenial with the staging-with-rebase target, but we should also add a Focal-specific CI target to ensure deb tests are passing for Focal as well.

We will probably want to have this run on a trigger similar to how the Xenial tests are run, since missing security updates to the build container will return a test failure.

One example below shows how it's done for Xenial tests:

https://github.com/freedomofpress/securedrop/blob/develop/.circleci/config.yml#L263-L278