GitLab

If dependencies between a platform's build and test jobs are not
specified, then artifacts from all platforms' build jobs are passed to
each test job. This is not what we want!

https://docs.gitlab.com/ee/ci/yaml/#dependencies

GitLab's shared runners are on DO 4GB instances, which should have 2
core processors.

These are mostly needed to pass between stages, but they might perhaps
be used for a bisection if necessary.

GitLab supports specifying directories as artifacts, so hopefully
letting GitLab archive it will be a bit faster.