ci: Enable Cirrus CI integration

ci: Enable Cirrus CI integration

We use cirrus-run to trigger Cirrus CI jobs from GitLab CI jobs,
making it possible to extend our platform coverage to include
FreeBSD without having to maintain our own runners; additionally,
we'll be able to ditch Travis CI and, since results for Cirrus CI
jobs are reflected back to the GitLab CI jobs that triggered them,
we will be able to get all information from a single dashboard.

The FreeBSD and macOS job definitions can be improved further: for
example, we will want to enable caching to speed up builds, and
ultimately we should figure out a way to generate at least part of
them, notably the list of packages to be installed, using lcitool.
All of that will happen in later patches: for now, this is good
enough to start using Cirrus CI.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>