Otherwise, if the upstream mirror changes (rather than deletes) a file, then tarballs.nixos.org won't be used even if it has a copy of the original file, and so we'll get a hash mismatch.
Otherwise, if the upstream mirror changes (rather than deletes) a file, then tarballs.nixos.org won't be used even if it has a copy of the original file, and so we'll get a hash mismatch.