a supposed long-lost van Gogh painting is a fake, much to the chagrin of a notable New York art collector who’s fighting the determination in a new $300 million lawsuit.Stuart Pivar — who founded the New York Academy of Art with Andy Warhol in 1979 — says he found the work, which he believes is Vincent van Gogh’s “Auvers, 1890” at a country auction in March.Pivar has “every reason to believe [the painting] was an authentic work of art created by Vincent van Gogh,” his Manhattan Supreme Court