Refactor CI to run in fresh build box

[fdupress]

This refactors the CI jobs so compilation and checks (including external CI) run in a fresh build box. This will help ensure that changes to the box configuration (new dependencies, prover updates, prover additions, ...) are reflected quickly in the CI environment and can be leveraged in external proofs right away.

The workflow is now:

1. Build base and build boxes, then save them as an artefact;
2. Recover the build box, then run the compilation check;
3. Recover the build box, then compile EasyCrypt and run a matrix check; (this recompiles EC 3 times in parallel, and likely needs changed)
4. Recover the build box, then compile EasyCrypt and run the external matrix checks (this recompiles EC as many times as we have external checks, and likely needs changed)
5. Build the test box, push the base and build boxes (all pushes) and the test box (release branches and tags)

The nix compilation check is unchanged, notification fires after all succeed. Documentation is built as normal.