We need a VM for both the compose-dry-run and the compose-base tests
thus to avoid spawning two VMs in Zuul, let's keep everything in a
single job to save money.
Test composing the base variant for all PRs. We don't compose all
variants yet as some fail in the Zuul infrastructure for an unknown
reason.
We only compose the base variant as it is the smallest and should give
enough assurance about the builds while not delaying PR merging too
much.
See: https://pagure.io/workstation-ostree-config/pull-request/405
