“The question in this case is not a close one,” Steidel Wall wrote. “The Executive Branch here is seeking access to records belonging to, and in the custody of, the Federal Government itself, not only in order to investigate whether those records were handled in an unlawful manner but also, as the National Security Division explained, to ‘conduct an assessment of the potential damage resulting from the apparent manner in which these materials were stored and transported and take any necessary remedial steps.’”