nixos/tests/containers-imperative: Fix eval

The commit c6f7d4367894047592cc412740f0c1f5b2ca2b59 changed the system
attribute to be below config.nixpkgs.localSystem, but the test still
uses the old attribute.

I have not tested whether the test actually succeeds but just checked
whether evaluation works and it evaluates successfully now.

Signed-off-by: aszlig <[e-mail]>

nixos/tests/containers-imperative.nix

···
       # container available within the VM, because we don't have network access.
       virtualisation.pathsInNixDB = let
         emptyContainer = import ../lib/eval-config.nix {
-          inherit (config.nixpkgs) system;
+          inherit (config.nixpkgs.localSystem) system;
           modules = lib.singleton {
             containers.foo.config = {};
           };