Refactor out common bits of CI pipeline?

[dc-mak]

#701 split up the CI into different pipelines, but there is a lot of common code between them. It's worth looking into whether or not using composite actions could reduce the duplication, and also how the opam cache works across the different workflows (presumably one per workflow right now?).

Especially because building cerberus and CN repeatedly seems unnecessary.

"Multiple workflow runs in a repository can share caches. A cache created for a branch in a workflow run can be accessed and restored from another workflow run for the same repository and branch." from here

Also this https://docs.github.com/en/actions/sharing-automations/reusing-workflows

Also this https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/passing-information-between-jobs

I think this means the current workflows are sharing a cache for opam? https://github.com/rems-project/cerberus/actions/caches