Update GitLab CI to OpenBSD 6.6
Since OpenBSD 6.6 is the current OpenBSD release, replace OpenBSD 6.5 GitLab CI jobs with their up-to-date counterparts.
As CI jobs for OpenBSD 6.6 will be run by a generalized libvirt executor rather than an OpenBSD-specific one, make the necessary tag and variable adjustments as well.
Closes #1305 (closed)
Edited by Michał Kępień