Update CI config to use the workflow setting

In !1 (merged) I tried to use the top-level workflow setting in .gitlab-ci.yml which is supported in newer versions of GitLab, but gitlri.lri.fr is still too out-of-date to support it.

This is reminder to re-enable this section of .gitlab-ci.yml once it's supported.

The advantage of this is it is a global setting for when a pipeline should be run at all; in the current version it is necessary to add an "only" property to each individual job.