ci(gitlab): run release job on default branch only

.gitlab-ci.yml

@@ -42,6 +42,8 @@ release:
     - pip install --user --requirement requirements.ci.txt
   script:
     - semantic-release --verbosity=DEBUG publish
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
 
 build container image:
   stage: build