Novice directors don’t usually get advice from the legendary David Lynch — unless you’re “Riverdale” star Madchen Amick, who makes her debut behind the camera for Wednesday night’s Season 4 finale. “David [Lynch] actually sent me a really sweet email the night before my first day of directing,” Amick, 49, tells The Post. “He said, ‘Just keep your eye on the doughnut, not the hole.’ I had his message printed out on my monitor every day to remind me to stay focused and get what I want and have fun with it.” Amick, who got her start playing Shelly Johnson on Lynch’s cult ABC hit “Twin Peaks,” currently co-stars as Alice Cooper on “Riverdale.” In Wednesday night’s episode, “Chapter Seventy Six: Killing Mr.