nixosTests.qemu-vm-external-disk-image: fix test

This test uses make-disk-image.nix and uses a bootloader to boot off the
resulting disk image, which requires the NixOS config to be switchable.

nixos/tests/qemu-vm-external-disk-image.nix

@@ -25,6 +25,8 @@ let
         "/".device = rootFsDevice;
       };
 
+      system.switch.enable = true;
+
       system.build.diskImage = import ../lib/make-disk-image.nix {
         inherit config lib pkgs;
         label = rootFslabel;