feat(skeleton): add workflow rules in .gitlab-ci.yml to avoid duplicated pipelines

cubicweb/skeleton/.gitlab-ci.yml.tmpl

 default:
   image: python
 
+# avoid running duplicate pipelines for both topic head and MR
+# see: https://docs.gitlab.com/ee/ci/yaml/#switch-between-branch-pipelines-and-merge-request-pipelines
+workflow:
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: '$CI_COMMIT_REF_NAME =~ /^topic\/.*/  && $CI_OPEN_MERGE_REQUESTS'
+      when: never
+    - when: always
+
 stages:
   - tests
   - after-tests