gitlab-ci: Reduce cost and latency of jobs

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.