restrict publishing to master branch

.gitlab-ci.yml

@@ -33,6 +33,8 @@ opam:
   script:
   # Send a trigger to the repository doing the work
   - curl --fail -X POST -F "token=$OPAM_UPDATE_SECRET" -F "ref=master" -F "variables[REPO]=${CI_PROJECT_URL}.git" -F "variables[REF]=$CI_COMMIT_REF_NAME" -F "variables[SHA]=$CI_COMMIT_SHA" -F "variables[NAME]=coq-stdpp" https://gitlab.mpi-sws.org/api/v3/projects/581/trigger/builds
+  only:
+  - master
 
 stdpp-coq8.7:
   <<: *template