How to detect the source that started a pipeline in GitLab CE 11.10?

I use an old version of GitLab and trying to make a pipeline. Feature branches are created using issues. So I want the pipeline to start only when a push event occurs, not when a branch is created from an issue.
I try to use “$CI_PIPELINE_SOURCE == “push”” condition in “only - variables” job section, but it seems that this doesn’t work.