OCaml layer: Changes with upcoming opam version 2.1

[bschommer]

With the next opam version 2.1 opam config var share will no longer work and be replaced by opam var share. I think one can support older and newer versions of opam by testing both and only failing if both return no result.

[smile13241324]

This sounds like changes would be necessary to the ocaml layer. As I am oblivious to ocaml I would ask the community to provide a PR for this. Support for both versions would be cool but not a must requirement in my opinion

[bschommer]

At the moment this isn't even a problem for users trying to use the new opam version since one can set the environment variable OPAMCLI to 2.0 to get the old behavior.