ACTOR CHARLTON Heston is in demand by government producers who value his top secret nuclear weapons security clearance as much as his Academy Award. For six years Heston has held the nation's highest security clearance because of his no-fee work narrating classified instructional films and videotapes for the government. And the actor knows what possessing the top security clearance means: Keep quiet. Heston has worked on at least six highly classified productions, the Energy Department says.