Add a way to configure the opam cache better

[samoht]

Right now it seems that the opam cache is saved after the compilation is successfully installed. This is great, but the usual (documented) workflows are then to do opam install . --deps-only --with-test in a separate step. The result of this is not cached (as far as I can tell). This is a bit wasteful!

Ideally it'd be great of the cache was indexed by the hash of the opam file + opam-repository HEADs, etc.

Or just add more control to when the cache is saved (and if we use a cache at all for users who are already using something like actions/cache/[save|restore]@v4