ocaml-s3, ocaml-scaleway: add eio.unix to lib/dune
Both .mli files reference Eio_unix.Stdenv.base in their public types.
Without eio.unix in (libraries ...) the cmi for Eio_unix isn't on the
include path and downstream consumers see 'Unbound module Eio_unix'.
Surfaced after the Dead_lib pass pruned the deps list, which made the
missing eio.unix visible.