fix(workflow): disable MR pipeline from prod & integ branches

Today creating a backmerge MR from prod or integ branch to any other branch triggers a pipeline
even though no change has been pushed.
Ultimately those pipelines might (re)trigger integration or production environment deployment and/or packages publication.
That might have unexpected consequences.
This change simply disables MR pipeline from prod & integ branches, enforcing branch pipelines only (i.e. when a true commit is pushed).