ci: Fetch all user cerbero branches when matching branch names

ci: Fetch all user cerbero branches when matching branch names

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()]:
```