wcow: support OCI-worker for Windows

[profnandaa]

Currently, only containerd worker is supported and therefore there is a hard dependency on containerd for buildkitd to run. On the Linux end, you have a vanilla option where you can run buildkitd with only runc using the OCI worker. Actually, this is the default for Linux, you set --oci-worker=false --containerd-worker=true to use the containerd worker.

However, on Windows, containerd is the only supported worker, and therefore the default.