GitLab CI: allow forcing to run a pipeline by setting $CI_FORCE_RUN

This is useful for example when someone without commit access created a MR for a branch in their own repository. In this case, we don't get Jenkins nor GitLab CI results, so usually we will push a copy of the branch to the main repo. This will give us Jenkins results, but not GitLab CI, because the copy of the branch has no MR.

With this change, one can manually trigger a GitLab CI pipeline run for a branch that has no corresponding MR, setting the $CI_FORCE_RUN pipeline variable.