fix(stargz): set default root path

This PR adds the default root path parameter to stargz-snapshotter
extension as an argument: `--root.` It sets the path to
`/var/lib/containerd/io.containerd.snapshotter.v1.stargz` as it is a more
common place to find it. The snapshotter default path is defined in
stargz-snapshotter/cmd/containerd-stargz-grpc/main.go#L66 and points to
/var/lib/containerd-stargz-grpc.

Signed-off-by: David Peralta <david@daper.email>
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

container-runtime/stargz-snapshotter/stargz-snapshotter.yaml

@@ -7,6 +7,7 @@ container:
   entrypoint: ./containerd-stargz-grpc
   args:
     - --address=/var/run/containerd-stargz-grpc/containerd-stargz-grpc.sock
+    - --root=/var/lib/containerd/io.containerd.snapshotter.v1.stargz
     - --log-level=debug
   security:
     rootfsPropagation: shared