scripts: set CRIU_IMAGE_DIR when running scripts

When doing a restore for LXC, we store some other metadata (which bridge a veth
was on) in the image directory so that the restore script can correctly unlock
a network device and attach it to the right interface. This patch is needed so
that the script can find this metadata.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>