Make site-lib take priority before the OCaml stdlib directory in the default OCAMLPATH

[kit-ty-kate]

Proposed by @a2line in https://github.com/ocaml/opam-repository/issues/27712

[dra27]

The order of those in #52 wasn't a mistake - the idea was that the opam site-lib should never be able to override the compiler's META files.