The truck will be attracted forward towards the magnet, but the magnet will also be attracted backward towards the truck with equal force. The backward force will be transfered through the arm holding the magnet and the net force on the whole system will be 0. If there was no arm holding the truck and the magnet apart, the truck could move forward slightly and the magnet could move backward to meet it. (The magnet would move much more than the truck since they would still have equal force applying to them, and the truck’s mass is much higher than the magnet’s.)