Dune workspaces are broken by dune-built ocamlfind

Hi, when setting up alternative toolchains while having `ocamlfind.1.8.1+dune` installed through opam-overlays, `dune` is not available to find the `<opam-root>/lib/findlib.conf.d/` directory because `ocamlfind printconf` doesn't return an output for the configuration file path. 

