Fix nix-copy-closure test

Fixes

  client# error: size mismatch importing path ‘/nix/store/ywf5fihjlxwijm6ygh6s0a353b5yvq4d-libidn2-0.16’; expected 0, got 120264

This is mostly an artifact of the NixOS VM test environment, where the
Nix database doesn't contain hashes/sizes.

http://hydra.nixos.org/build/53537471