[bug] onCopySkipped called unexpectly on root node when using oras.Copy

[qweeah]

See below snippet, onCopySkipped (396-400) should not be invoked when destination target is a reference pusher, since line 406 does fetch the root node from proxy and push it to the destination, which is copying.

oras-go/copy.go

Lines 396 to 410 in f0a9c59

	if onCopySkipped != nil {
	if err := onCopySkipped(ctx, desc); err != nil {
	return err
	}
	}
	if !content.Equal(desc, root) {
	return nil
	}
	// enforce tagging when root is skipped
	if refPusher, ok := dst.(registry.ReferencePusher); ok {
	return copyCachedNodeWithReference(ctx, proxy, refPusher, desc, dstRef)
	}
	return dst.Tag(ctx, root, dstRef)
	}