Optimize Windows and LCOW snapshotters to only create scratch layer on the final snapshot

For LCOW currently we copy (or create) the scratch.vhdx for every single snapshot
so there ends up being a sandbox.vhdx in every directory seemingly unnecessarily. With the default scratch
size of 20GB the size on disk is about 17MB so there's a 17MB overhead per layer plus the time to copy the
file with every snapshot. Only the final sandbox.vhdx is actually used so this would be a nice little
optimization.

For WCOW we essentially do the exact same except copy the blank vhdx from the base layer.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

rootfs/apply.go

@@ -123,7 +123,7 @@ func applyLayers(ctx context.Context, layers []Layer, chain []digest.Digest, sn
 	)
 
 	for {
-		key = fmt.Sprintf("extract-%s %s", uniquePart(), chainID)
+		key = fmt.Sprintf(snapshots.UnpackKeyFormat, uniquePart(), chainID)
 
 		// Prepare snapshot with from parent, label as root
 		mounts, err = sn.Prepare(ctx, key, parent.String(), opts...)