GitLab

* fix(ci): change github mac runner version

Github Actions changes the macos-latest runner to use mac os 11 instead
of 10.15. See https://github.com/actions/virtual-environments/issues/4060

This broke some of our build process since it looks like ocamlopt is only available
on homebrew as an intel binary and not ARM. So for now pin to macos-10.15 runners

* Update tests as well