pkg: topkg doesn't build

[Alizter]

Building on the solution in #9013 for ocamlfind we have an error with building topkg:

       ocaml default/.pkg/topkg/target [_private] (exit 125)
(cd _build/.sandbox/fc0ec1450672eeebcf74d3ea055dadc2/_private/default/.pkg/topkg/source && /nix/store/bink43irwl4lp7fli0lh6j82zsrkycp5-ocaml-4.14.1/bin/ocaml pkg/pkg.ml build --pkg-name topkg --dev-pkg false)
Cannot find file ../target/lib/findlib/findlib.cma.
Exception: Compenv.Exit_with_status 125.
-> required by _build/_private/default/.pkg/topkg/target/cookie

I imagine that this means some configuration in findlib is missing.

[rgrinberg]

Yeah, this is a duplicate of the findlib issue.