fix: remove ocamlbuild from compilation process of `opam-lib`

this makes the package build on (debian wheezy)+(opam switch 4.01.0),
as with ocamlbuild it fails.

cf. https://github.com/ocaml/opam-repository/issues/1789

Also related to https://github.com/ocaml/ocaml.org/pull/440

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apparently it's better not to merge this one...