Conflict needed with older cmdliner versions

[mseri]

Appeared on a few opam-ci lower bound tests (e.g https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/5516a097c2586e34975c0260e7009bbfb7723df5/variant/opam-2.1,compilers,4.03,odoc.2.0.0,lower-bounds):

The error we saw is:

# + ocamlfind ocamlc -c -bin-annot -safe-string -package cmdliner -I src -I test -o src/logs_cli.cmi src/logs_cli.mli
# File "src/logs_cli.mli", line 15, characters 17-33:
# Error: Unbound type constructor Cmdliner.Arg.env

with the following downgrades (but the Arg.env type has been there for a while as far as I can see in logs_cli.mli)

  - downgrade cmdliner                 1.0.4 to 0.9.6     [uses ocamlbuild, ocamlfind]
  - downgrade logs                     0.7.0 to 0.6.3     [uses ocamlbuild, ocamlfind]