fix .test-refs-check-benches condition (#13906)

This check is meant to catch pipelines triggered by the scripts ci-linux
staging tests. The correct CI_PIPELINE_SOURCE for multi-project
pipelines such as this is "pipeline"; "parent_pipeline" is only set for
child pipelines triggered /within the same repo/.

cf https://docs.gitlab.com/ee/ci/jobs/job_control.html#common-if-clauses-for-rules