ci: fix pipeline run for merge requests

This way CI pipeline runs for branches and tags
and makes it show up in merge requests where
a branch is used as source branch.

Makes all jobs show up in merge request CI
indicator (not just the whitespace job) and
only runs a single CI pipeline, without the
additional detached pipeline.

https://docs.gitlab.com/ce/ci/yaml/#workflowrules-templates
4 jobs for ci-fix-pipeline-for-merge-requests in 5 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #11096
docker
autoconf

00:04:58

passed #11097
docker
cmake

00:02:01

passed #11098
docker
meson

00:02:22

 
  Test
passed #11099
docker
whitespace

00:00:05