treewide: pass system argument to eval-config.nix

Calling `eval-config.nix` without a `system` from a Nix flake fails with
`error: attribute 'currentSystem' missing` since #230523. Setting
`system = null` removes the use of `currentSystem` and instead uses the
value from the `nixpkgs` module.

nixos/modules/virtualisation/nixos-containers.nix

@@ -515,6 +515,10 @@ in
                     in [ extraConfig ] ++ (map (x: x.value) defs);
                   prefix = [ "containers" name ];
                   inherit (config) specialArgs;
+
+                  # The system is inherited from the host above.
+                  # Set it to null, to remove the "legacy" entrypoint's non-hermetic default.
+                  system = null;
                 }).config;
               };
             };