Recent versions of Async no longer shadow Sys.getenv_exn, so remove `Core.Std` prefix
Recent versions of Async no longer shadow Sys.getenv_exn, so remove `Core.Std` prefix

inhabitedtype/ocaml-cohttp,fdopen/ocaml-cohttp,inhabitedtype/ocaml-cohttp,mor1/ocaml-cohttp,Drup/ocaml-cohttp,marklrh/ocaml-cohttp,marklrh/ocaml-cohttp,inhabitedtype/ocaml-cohttp,marklrh/ocaml-cohttp,fdopen/ocaml-cohttp,Drup/ocaml-cohttp,edwintorok/ocaml-cohttp,edwintorok/ocaml-cohttp,mor1/ocaml-cohttp,mor1/ocaml-cohttp,fdopen/ocaml-cohttp,Drup/ocaml-cohttp,dsheets/ocaml-cohttp,dsheets/ocaml-cohttp,dsheets/ocaml-cohttp,edwintorok/ocaml-cohttp