workflow: faster workflow + lower bound on ocaml
workflow: faster workflow + lower bound on ocaml

The old workflow was a little too lax on the Ocaml versioning, as our coq-bits
dependency is pretty old.

This also switches to a much faster setup-ocaml action, going down from 4+
minutes to < 2 minutes.  However, it is experimental, and time will tell
whether we stay happy with it.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script