Add a way to configure the opam cache better #1025
Description
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