Limit triggers for OpenBSD system test jobs

When a GitLab CI runner is not under load, a single OpenBSD system test job completes in about 12 minutes, which is considered decent. However, such jobs are usually multiplexed with other system test jobs on the same host, which causes each of them to take even 40 minutes to complete. Taking retries into account, this is completely unacceptable for everyday use, so only start OpenBSD system test jobs for pipelines created through GitLab's web interface and for pipelines created for Git tags.