nixos.tests.containers-imperative: fix impurity failure

I can't claim to fully understand this, but the change seems safe,
and it fixes the test on my machine.

nixos/tests/containers-imperative.nix

···
             containers.foo.config = {};
           };
         };
-      in [ pkgs.stdenv emptyContainer.config.containers.foo.path ];
+      in [ pkgs.stdenv emptyContainer.config.containers.foo.path pkgs.libxslt ];
     };
 
   testScript =