apertis-pkg-merge-upstream-to-downstreams: cancel pipelines created through the creation of a branch

Due to GitLab limitation (https://gitlab.com/gitlab-org/gitlab/-/issues/26422

),
it's currently impossible to avoid to trigger a pipeline when creating a
branch with the API. Since these useless pipelines are wasting
GitLab and OBS ressources (and devs' time), as a workaround for now
we can cancel them.

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