If there's no $OCAMLPATH envvar set, dune build will fail because it doesn't rely on findlib.conf in default context.
$OCAMLPATH is only supposed to be an additional search path for package directories.

I stumbled upon this bug while working on making esy's build plan use findlib configs instead of environment variables. The main purpose of it, besides cleaner env, is laying out the ground work for cross-target compilation.

Unless you have other suggestions on which direction to follow, I think dune should respect OCAMLFIND_CONF env var.

Cc: @rgrinberg