WCOW: support FROM scratch for Windows #5264

Out of scope for this PR: Is the FROM scratch not working at all in windows? I think in buildkit we should make it work. It should fail if you need to create image/container on top of FROM scratch stage, but if you just use docker buildx build -o out --target=binaries . then that should work in windows as well just to get the artifacts without base image.

Originally posted by @tonistiigi in #5253 (comment)