Lintian's Gitlab CI pipeline stages

[Felix Lechner <felix.lechner@lease-up.com>]

Hi Chris,

I noticed that you previously tried to combine the two CI pipeline
stages, but then reverted. The timeout is now three hours. Would you
mind if I combine them again?

My reasons are:

    1. When test packages fail to build, other pipelines may still complete.
    2. When a pipeline needs to build few or no test packages, it
completes faster.
    3. The resolution of debhelper-compat (= 13) from stable-backports
only needs to be fixed in one place.

Your guidance would be appreciated.

Kind regards
Felix Lechner