gitlab-ci: limit the automatic CI to master and MRs

Runs on random other branches (stables RCs, personal forks) can still be
triggered manually via the web interface, or an app using the API.

This should massively help with the current voracious state of our CI.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>

.gitlab-ci.yml

@@ -63,6 +63,9 @@ containers:ubuntu:
 .build:
   image: $UBUNTU_IMAGE
   stage: build+test
+  only:
+    - master
+    - merge_requests
   artifacts:
     when: on_failure
     untracked: true