[ci] improve CI pipeline workflow rules

.gitlab-ci.yml

 stages:
     - "test"
 
+# Override the default workflow rules to only run pipelines on the master
+# branch, merge requests, or branches that contain the string 'ci/'
+workflow:
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
+    - if: $CI_COMMIT_TAG
+    - if: $CI_COMMIT_REF_NAME == "master" || $CI_COMMIT_REF_NAME =~ /^(.+\/)?ci[-\/].+/
 
 .test: &test
     before_script: