data-fetch-downstream: Address gitlab pagination warning

The python-gitlab package started warning about some ambiguous usage of their API:

UserWarning: Calling a `list()` method without specifying `all=True`
or `as_list=False` will return a maximum of 1 items. Your query
returned 1 of 3 items. See
https://python-gitlab.readthedocs.io/en/v3.5.0/api-usage.html#pagination
for more details. If this was done intentionally, then this warning
can be supressed by adding the argument `all=False` to the `list()` call.
(python-gitlab: /usr/lib/python3/dist-packages/gitlab/client.py:879)

We actually do that on purpose to only get the latest pipeline for each branch, so add all=False.