No no
Using the ground wire for a neutral is ALWAYS a NO NO... Let me explain why. Ground wires or (grounding conductor) is exactly that, a grounding conductor. Grounding conductors are used to protect against electrical shock. If you use this wire to complete a 120v circuit, you are using it as a current carrying conductor. If the circuit or light is on and someone lifts the ground at the panel or at any junction box they can get shocked or even electrocuted. The grounding conductor is used to ground any metal or conductive material that may become energized. That is why sparks happen when you touch a current carrying conductor to a metal housing on a box, light, or the like. An enclosure without a proper ground can become energized and shock the crap out of you or worse..... The two pole breaker will still trip if only one leg has overload or short. But, what should be done is,,, change the breaker to a single pole sized for the circut and pull a properly sized white wire in by using one of the wires on the existing two pole breaker and terminate it to the neutral bar and to the common on the light fixture. Or you could always have a professional electrician do it.