packaging-check-settings: Improve the gitlab-rulez checks
Move the call to gitlab-rulez to a separate job to shorten a bit the
already extra-long packaging-data-fetch-downstream job, and shift it
to the check stage as it seems more appropriate.
This also causes the pipeline to properly honor the $GITLAB_RULES_URL
variable, as it stops using the hardcoded
gitlab.apertis.org/infrastructure/apertis-infrastructure URL in
bin/packaging-data-fetch-downstream.
Reporting is also improved by showing details about the incorrect
settings by using the recently introduced --output json option in
gitlab-rulez, currently only available starting from Debian Trixie.
Edited by Emanuele Aina