switch Hub OCaml containers to point to opam-repository:2.0.0 branch

[avsm]

Right now every container uses the opam-repository master branch, which has the opam1 metadata. This allows (e.g.) opam-ci to use the opam-1 git revs to track PR tests, but increases the time it takes to do updates in the containers due to the opam1->2 conversion.

At some point, we need to coordinate with the datakit-ci and switch the containers to use the 2.0 branch natively, and test the conversion scripts separately.