Rubén Rocha Moya, the Mexican governor indicted by the United States this week, said Friday night that he would temporarily step down from his post, expanding the political fallout from accusations that he aided a powerful drug cartel for years. Mr. Rocha, 76, said in a two-minute recorded statement late Friday that he was innocent but would take “a temporary leave of absence” as governor of the Mexican state of Sinaloa to focus on defending himself from the accusations.