Target a particular compiler PR or branch under multicoretests-ci

[jmid]

Thanks to #366 we can focus on one particular test across both GitHub actions and multicoretests-ci.
Being able to target the test suite at a particular compiler PR or branch is another useful feature, which is currently limited to GitHub actions. To lift this restriction, more of the workflow logic will have to move into dune files and/or into multicoretests-ci.

Ideally, we would like to be able to test a PR or feature branch across both CI systems by changing only a couple of lines of (dune?) code.