party in its opening brief to challenge an alternate ground for a district court's ruling . . . waives that challenge." United States ex rel. Ubl v. IIF Data Solutions, 650 F.3d 445, 456 (4th Cir. 2011) (quoting Rodriguez v. Hayes, 591 F.3d 1105, 1118 n.6 (9th Cir. 2010))). The workers contend first, and we agree