It's not clear why removing this makes things work. I've submitted
PRs that passed CI with fetch-depth=1. Maybe it only fails when
PRs are submitted from external contributors?
Either way, for now, we remove this and absorb the extra cost in
order to get PRs passing CI again.
PR #1501
Basically, matrix.os needs to be defined for every build. We
were commenting out some of the builds in order to debug
CI in the `include` section, but we also need to comment them
out in the `build section.
