CI improvements

[dtebbs]

Some notes about possible CI improvements to consider

• reduce duplicated code with something such as composite actions
  • these don't yet support calling into other actions, although that appears to be the plan
  • Next Steps for Fully Functioning Composite Actions actions/runner#646
• reduce duplicated builds
  • consider progresively building (and in some caches caching) docker images and passing between jobs as artifacts
    • build (and cache) base image with dependencies installed
    • from base image, build libzeth and prover in various configurations
    • from built images
      • run units tests
      • run client tests
      • run integration tests
      • ...
    • avoids rebuilding everything, but introduces cost of exporting and passign docker images
    • many ci build scripts can be replaced by docker files
    • cannot necessarily be used for mac