Conform nydus cache image to OCI spec

[imeoer]

1. Using nerdctl pull --snapshotter nydus localhost/busybox:nydus-cache to pull the nydus cache image will result in the error number of layers and diffIDs don't match: 2!=0.
2. If the source image has only one platform, generate the nydus cache image as a manifest instead of a manifest list.

While we don't usually pull the nydus cache image, it's preferred that cache images conform to the OCI spec wherever possible.

[imeoer]

cc @PerseidMeteor