Hi Guix!

This small series updates opam and dune. Opam was surprisingly
difficult to update, after the switch to dune, but I had to update it
because the previous version we had doesn't build with the new dune.

The most difficult part was making the tests pass, as they require
network access. For some of them, I was able to download the files in
advance (see opam-repo in the inputs), and change the tests to use
that, fix shebangs, ... I still had to disable three tests: one that
runs opam and let it clone a git repository, one that fails only
because of a difference in text formating (it's expected to be on one
line, but actually is nicely formated), and one that compares a
generated dune file with the one that was just modified.

also ocaml-markup fails with the new dune, so I updated it to the
latest version.