amend! Merge the CI workflows. Stop using setup-ocaml.

Merge the CI workflows and stop using `setup-ocaml`

Merge the two CI main workflows, namely `common.yml` and
`msvc-common.yml` into a single `common.yml` workflow that uses neither
the `setup-ocaml` action nor `OPAM` so that:

- the workflow has reduced dependencies and a smaller footprint,
- the compiler configuration can be tuned (to disable features that are
  not relevant to the multicoretests, such as manpages),
- it gets easier to test any branch of any fork of the compiler,
- it brings the workflow really close to a version that can be pushed to
  the compiler repository.

The logic of the workflow is largely extracted to a `runner.sh` script
so that:

- it is easier to maintain, as it is a regular script, instead of being
  embedded into a different syntax, especially with the issue of
  portability to Windows due to line endings,
- it is easy to reuse that logic for different setups than regular GH
  Action runners, such as VMs, other infrastructures, etc.

Extra notes:

- the actual test steps are currently not triggered through the
  `runner.sh` script, in the unlikely case this could make some
  difference on Windows to have an `sh` involved somewhere in the stack,
- the new common-workflow default is to test `trunk` rather than 5.0.0,
  which seems a better default.