macOS: use nerdbox as the default runtime, erofs as the default snapshotter

[AkihiroSuda]

When nerdctl is directly executed on macOS, it should use nerdbox+erofs (containerd/project#154) as the default for ease of running Linux containers.

This proposal does not affect Lima; nerdctl.lima will continue to use runc and overlayfs.

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Blockers:

• [macOS + erofs] nerdctl pull fails with failed to mount /private/var/lib/containerd/tmpmounts/containerd-mount4013524920: not implemented #4570
• [macOS + nerdbox] nerdctl run fails with not implemented #4571