Honor the target distribution (according to d/changelog)

When building packages targeting a specific Debian release (i.e. not unstable), we must set the RELEASE environment variable accordingly and ensure the build environment/container will be based on this specific release.

As this couldn't be done in a simple way, this MR reworks our main pipeline definition (mobian-ci.yml) so it generates then triggers a child pipeline instead of being a "static" CI pipeline. This gives us more flexibility in order to change variables/includes/jobs depending on the packages's target distro.