Update GitLab CI to FreeBSD 12.1

With isc-projects/images!25 merged, we can now also move FreeBSD CI jobs to libvirt-based executors. Since no FreeBSD jail was prepared for version 12.1, we need to do both things (12.1 update & moving to libvirt) in the same merge request.

However, I would like to postpone merging this until after we get the %March 2020 (9.11.17, 9.16.1, 9.17.0) releases tagged as some system tests (dnssec, dnstap, and resolver in particular) seem to be more prone to timing issues under libvirt than on physical hardware (I guess that was expected). Postponing the merge gives us time to hopefully address the underlying issues in the affected tests without messing up the %March 2020 (9.11.17, 9.16.1, 9.17.0) release schedule.