check-dependent-cumulus should only be executed for PRs (#11693)

the script executed by check-dependent-cumulus only works for PRs, as shown in https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/1630771#L87, which comes from https://github.com/paritytech/pipeline-scripts/blob/3ad10ddc0d985ef5326974a1143229c6429befab/check_dependent_project.sh#L443