Draft: CI: use 'workflow' to avoid duplicate pipelines

In our CI setup, duplicate pipelines are started when pushing to a branch that is attached to an open Merge Request.

For example:

This happens because when a MR is created, a new commit to the attached branch triggers one pipeline for the open MR and one for the branch itself.

The CERN GitLab team suggested me to take a look at those pages:

• https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html#two-pipelines-when-pushing-to-a-branch
• https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines

In this MR, I will try to configure our CI to avoid duplicate pipelines.