Reupload Github's archive as a custom archive to avoid Github's archive recomputing issue

[kit-ty-kate]

Github sometimes recompute archives linked to tags and thus can cause problem to opam-repository when that happens. Currently opam-publish just uses the generated archive given by github, however one fix for that would be to fetch the archive and then re-upload it as an extra archive to avoid Github messing with it. Dune-release already uses this method, I think it would be a good feature to have in opam-publish as well for packages that don't use dune or don't want to use dune-release.