nixos/nixos-build-vms: fix eval

Within #193485 (and the previous changes) the internal structure of the
testing driver was changed. Since then, `makeTest` returns the
attributes for the VM test(s) (including `driverInteractive`) inside a
sub-attribute called `test`, so without this change running
`nixos-build-vms` would fail like this:

error: attribute 'driverInteractive' missing

Changed files
+1 -1
nixos
modules
installer
tools
nixos-build-vms
+1 -1
nixos/modules/installer/tools/nixos-build-vms/build-vms.nix
···
inherit system pkgs;
};
-
interactiveDriver = (testing.makeTest { inherit nodes; name = "network"; testScript = "start_all(); join_all();"; }).driverInteractive;
+
interactiveDriver = (testing.makeTest { inherit nodes; name = "network"; testScript = "start_all(); join_all();"; }).test.driverInteractive;
in