gitlab: support for FreeBSD 12, 13 and macOS 11 via cirrus-run

gitlab: support for FreeBSD 12, 13 and macOS 11 via cirrus-run

This adds support for running 4 jobs via Cirrus CI runners:

 * FreeBSD 12
 * FreeBSD 13
 * macOS 11 with default XCode
 * macOS 11 with latest XCode

The gitlab job uses a container published by the libvirt-ci
project (https://gitlab.com/libvirt/libvirt-ci) that contains
the 'cirrus-run' command. This accepts a short yaml file that
describes a single Cirrus CI job, runs it using the Cirrus CI
REST API, and reports any output to the console.

In this way Cirrus CI is effectively working as an indirect
custom runner for GitLab CI pipelines. The key benefit is that
Cirrus CI job results affect the GitLab CI pipeline result and
so the user only has look at one CI dashboard.

[AJB: remove $TEMPORARILY_DISABLED condition, s/py37/py38/]

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210625172211.451010-3-berrange@redhat.com>
Message-Id: <20210709143005.1554-8-alex.bennee@linaro.org>