Zac Efron is turning his stay in Australia into a working vacation after signing on for survival thriller Gold.The movie, about two strangers who discover the biggest gold nugget ever found in the middle of the desert, was co-written by Animal Kingdom actor Anthony Hayes, who will direct and also co-star.“This is an exciting, gripping and timely tale about greed, humanity, who we are, what we’ve done to the world and where we are heading if we aren’t careful,” Hayes shares in a statement.“To