.gitlab-ci.yml: Launch pipelines on merge requests only

Consistency with doc/.gitlab-ci.yml.
Otherwise we have a pipeline on the branch itself and one pipeline on the merge request.

Fix vdj#1020
Status Job ID Name Coverage
  Tgz Release
passed #552036
doc
build_doc

00:00:45

 
  Deploy Prod
manual #552038
allowed to fail manual
deploy_doc