Fix Mac overlay
I recently found out (thanks to @awkure) that the Nix build is broken on Mac. This MR fixes the build by applying the ocaml-overlay from tezos-packaging in a better way (with extend) and moving the Mac overlay to the ocaml-overlay.
I recently found out (thanks to @awkure) that the Nix build is broken on Mac. This MR fixes the build by applying the ocaml-overlay from tezos-packaging in a better way (with extend) and moving the Mac overlay to the ocaml-overlay.