Add the `lightweight` tag to run jobs on the GitLab runners set up for
jobs that do not need large amounts of resources (max 2GB of RAM) and do
not need privileged containers.

The former makes jobs cheaper because they user smaller VM instance
types, the latter improves latency as there's no strong need to spin
separate VMs to keep each job isolated and existing instances can be
re-used instead.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>