`Env_binding` equal operator can't be reached

Using `OpamLexer`, `a = b` is always parsed as an `Relop ('Eq, ident "a", ident "b")`.
Cf. [lexer](https://github.com/ocaml/opam-file-format/blob/master/src/opamLexer.mll#L122) & [parser](https://github.com/ocaml/opam-file-format/blob/master/src/opamBaseParser.mly#L85)