Motivation for this change

#34931: it is not convenient to use utop, as dynamically linked shared libraries cannot be found.

These libraries are searched in the directories listed in the CAML_LD_LIBRARY_PATH environment variable (see https://github.com/diml/utop#common-error for details).

This PR adds a setup-hook to the utop derivation; this hook automatically populates the CAML_LD_LIBRARY_PATH variable. Thus, in a shell with utop, findlib and some OCaml libraries with DLLs, that variable will be appropriately set, and dynamic loading (aka #require "…";;) will work.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.