Fixes CI on coordinated merges when the user has more than 20 branches
in their fork, which will happen very easily since new forks will
always have all the branches of the original remote.
```
ci/gitlab/trigger_cerbero_pipeline.py:48: UserWarning: Calling a `list()` method without specifying `get_all=True` or `iterator=True` will return a maximum of 20 items. Your query returned 20 of 37 items. See https://python-gitlab.readthedocs.io/en/v3.9.0/api-usage.html#pagination for more details. If this was done intentionally, then this warning can be supressed by adding the argument `get_all=False` to the `list()` call. (python-gitlab: /usr/local/lib/python3.7/site-packages/gitlab/client.py:979)
if os.environ["CI_COMMIT_REF_NAME"] in [b.name for b in cerbero.branches.list()]:
```
When GStreamer is forked into a private GitLab instance we should
trigger cerbero into that private instance too. Otherwise the token
won't be accepted.
Also do not hardcode the cerbero project ID because it is instance
specific.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1056>
