See if we can encode the `dune.native` trick with Dune 3

[Niols]

cf https://github.com/ocaml/opam-repository/pull/23667/files#r1166318717 for some more context

[Niols]

Debian stable only ships with Dune 2.7 at the moment, though.

[Niols]

But it has been a while that Dune supports configuration via an OCaml executable generating a Dune file, so we could do that instead.