What are they building in there? HBO’s Cube takes shape. If you’ve been up at the intersection of 18th Street and Columbia Road NW today, you’ve no doubt noticed the freestanding scaffolding and truck parked there on the plaza. No, it’s not construction beginning on the controversial plaza sculpture (that’s been tabled for the moment), but rather a temporary art installation — or advertisement, depending upon your level of cynicism — from HBO and…