depend on the checkouts of ocaml/opam

[avsm]

We really ought to track the changesets of ocaml/opam when building the opam binary. For example merging this ocaml/opam#4648 should have triggered off a rebuild without requiring manual intervention.

(this is likely to be more important as we're speeding up the opam release process post 2.1.0)

[shonfeder]

This applies to services using the dev version of opam, and getting this in place could reduce our manual work and improve stability.