Makefile: Set OPAMCLI to 2.0

This removes the warning about `opam config var`. Setting OPAMCLI in
this way is the correct thing to do if we want to continue supporting
opam 2.0.

If we decide to require opam 2.1+ then all `opam var` invocations should
become `opam --cli=2.1 var`, as per the opam CLI versioning spec:

https://github.com/ocaml/opam/wiki/Spec-for-opam-CLI-versioning