Limit triggers for OpenBSD system test jobs

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.

(cherry picked from commit 603e04563b65d230651c6861f4459c0ebdcd8fc5)