A former pizza-slinger accused of posing as an FBI agent to try to spring Luigi Mangione was hauled back to the same Brooklyn jail as the accused killer Tuesday after pleading not guilty to his half-baked plot. Mark Anderson, 36, was swiftly returned to the Metropolitan Detention Center, the same lockup where Mangione awaits trial for allegedly executing UnitedHealthcare CEO Brian Thompson, after the feds cited his long rap sheet and argued other reasons at a hearing in Brooklyn federal court.