A conventional documentary about idiosyncratic post-punk icon Nick Cave wouldn’t be right. Fortunately, codirectors Iain Forsyth and Jane Pollard tossed the biographical instruction manual in the trash and started fresh. 20,000 Days on Earth—Cave’s age at the time of filming—plays like a living scrapbook. Cave worked with the filmmakers, becoming a collaborator as much as a subject, recording and performing songs from 2013’s Push the Sky Away…