But why build the VM for every release then? What is it that needs to be cross compiled if you're just copying a VM build into a tarball? Why not copy an existing build of it in?
The VM is not built for every release, it is copied into the release. The fact that it isn't built for each release is why cross-compilation of releases is not possible.
I must be an idiot because I still don't get it. If the VM is just copied into the release, not built, why can't you do that on macOS for deployment on Linux by copying a Linux version in? Why do you need to do it in a VM or Linux build server?