Fix using `odoc_driver` to build `odoc` Quoting the docs of the Findlib library: > * Furthermore, the environment variables OCAMLPATH, OCAMLFIND_DESTDIR, > * OCAMLFIND_COMMANDS, OCAMLFIND_IGNORE_DUPS_IN, and CAMLLIB are interpreted. > * By default, the function takes > * the values found in the environment, but you can pass different values > * using the [env_*] arguments. By setting these values to empty strings > * they are no longer considered. So when we do ``` dune exec -- odoc_driver -p odoc ``` findlib finds the `odoc` library in `_build/install` in some part of the codebase, in `<opam switch>/lib/odoc` in some other parts of the code, which results in the docs for `odoc` not being built. This changes to consistently find the `<opam switch>/lib/odoc`.