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, that no waiver occurred because their arguments in the openin