apertis-pkg-merge-upstream-to-downstreams: Don't trigger a pipeline in branch context

Only the pipeline in merge request context is relevant here.
"ci.skip" only affect branch pipelines and not merge request pipelines.
See https://docs.gitlab.com/ee/gitlab-basics/add-file.html#push-options-for-gitlab-cicd



Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>