Flush disk writes before reboot

This commit introduces a `sync` call before the install script exits.

The writes that this script does during setup do not reach persistent
storage until the kernel decides to flush them.  The reboot sequence
might drop those pending writes on the floor.  To mitigate that
possibility (but not fully eliminate it), the script now calls `sync`,
which waits until the kernel has flushed its write cache.

auto_install/install.sh

@@ -1371,6 +1371,10 @@ main() {
             $SUDO systemctl start openvpn.service
             ;;
     esac
+    
+    # Ensure that cached writes reach persistent storage
+    echo "::: Flushing writes to disk..."
+    sync
 
     echo "::: done."