`skopeo` allows replacing `docker` and `oras` with a single tool that
behaves in a way that's a little more intuitive for us (we build the
directory structure on disk and upload that). It seems to be better at
preserving our metadata, too.
Take the opportunity for writing out more metadata ourselves to split
out more logic into separate functions.
While we're here, fix the `download_strategy` timeout handling for
GitHub Packages.
