nixos/tests/packagekit: fix test machine evaluation
nixos/tests/packagekit: fix test machine evaluation

aa22be179a4dfb9633089ebc7c65c6d6c18a83d5 dropped the backend setting
which was used in the test, breaking evaluation of the test in the
process. Kind of defeats the purpose of a test if it isn't executed
before merging a change to a module…

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs