Check if pipeline already running before starting new one when merging security MR

During a security release, RMs sometimes have to run the run release merge --security --default-branch multiple times in a short duration. Currently, each time the command is executed, a new MWPS pipeline is started for open MRs.

When merging the default branch MR of a security issue, release-tools starts a new pipeline and sets MWPS. However, if a pipeline is already ongoing, we should not start a new one since pipelines can take an hour to complete.

The change probably needs to be in security/merge_when_pipeline_succeeds_service.rb#L13.