Local pipelines in debian/apertis/local-gitlab-ci.yml

If debian/apertis/local-gitlab-ci.yml exists, use it to trigger child pipelines to run some local logic. This give people the chance to run their own jobs before the actual packaging workflow.

Custom jobs like the one in base-files stopped working when we switched to the external CI config. This MR provides a place where those jobs can be ported with minimal effort.