[containerd-snapshotter] add support to prune build cache type=regular

Description

do:

1. docker build
2. docker system prune --all --volumes
3. docker buildx prune -a -f
4. docker system df -v

still show build cache with cache type=regular

/etc/docker/daemon.json

{
  "data-root": "/mnt/other_disk/docker_store",
  "default-address-pools": [
    {
      "base": "100.100.0.0/12",
      "size": 16
    }
  ],
  "features": {
    "containerd-snapshotter": true
  }
}

/etc/containerd/config.toml

disabled_plugins = ["cri"]
root = "/mnt/other_disk/containerd-data"

Reproduce the issue with the commands and configuration shown, especially the containerd snapshotter settings in daemon.json and config.toml. Start by tracing how buildx prune and Docker system df classify and remove cache type=regular; done when the regular cache is pruned and no longer appears in the disk-usage output.