Prep for adding more CI targets. Building dbg and opt in the same job
(as standard) simplifies caching, debugging and artifact handling. With
ccache it should not take much longer either. Also removes the need to
re-configure in the test job.
GitHub Actions starts the jobs earlier in the list first. This change
has the effect of starting the few longer running jobs (those on ubuntu
20.04) first.
GitHub Actions allow a total 5 GB of cache storage space per repository,
after which it will evict old caches. Tweaked cache sizes so master + a
few PRs can fit at the same time. Don't cache coverage builds as they
run weekly and with additional compiler options, so their caches would
likely be state anyway.
