Paul McCartney turned down being in the hot seat for the Netflix roast , but he'll serve as a musical guest on SNL this weekend for the season finale. It'll be Sir Paul's fifth time as musical guest, but he is a longtime friend of creator/showrunner Lorne Michaels and has made several additional cameos over the years. It'll be former cast member Ferrell's sixth time hosting. The promo clip is here now. McCartney, Ferrell, and cast member Marcello Hernández joke that people will only be tuning in to see McCartney's performance. The Beatle also quips that he's a big fan of Hernández's mom — a callback to last week's Matt Damon monologue for Mother's Day. Hernández asked McCartney what it's like being a knight (he was knighted by Queen Elizabeth II in 1997 for his "services to music"), and McCartney nonchalantly mentions that he once slayed a dragon. The clip ends with the trio singing "Despacito" (the Justin Bieber remix, to be clear).…