Do not run OpenBSD system test jobs for tags

OpenBSD virtual machines seem to affected particularly badly by other activity happening on the host. This causes trouble around release time: when multiple tags are pushed to the repository, a large number of jobs is started concurrently on all CI runners. In extreme cases, this causes the system test suite to run for about an hour (!) on OpenBSD VMs, with multiple tests failing. We investigated the test artifacts for all such cases in the past and the outcome was always the same: test failures were caused by extremely slow I/O on the guest. We tried various tricks to work around this problem, but nothing helped.

Given the above, stop running OpenBSD system test jobs for pending BIND releases to prevent the results of these jobs from affecting the assessment of a given release's readiness for publication. This change does not affect OpenBSD build jobs. OpenBSD system test jobs will still be run for scheduled and web-requested pipelines, to make sure we catch any severe issues with test code on that platform sooner or later.