nixos/nixos-build-vms: use pkgs.qemu for virtualisation

When I test a change e.g. in the module system manually, I usually use
`nixos-build-vms(8)` which also gives me a QEMU window where I can play
around in the freshly built VM.

It seems as this has changed recently when the default package for
non-interactive VM tests using the same framework was switched to
`pkgs.qemu_test` to reduce the closure size. While this is a reasonable
decision for our CI tests, I think that you really want a QEMU window of
the VM by default when using `nixos-build-vms(8)`.

[1] bc2188b083

nixos/modules/testing/test-instrumentation.nix

@@ -53,7 +53,7 @@ with import ../../lib/qemu-flags.nix { inherit pkgs; };
     #       or declare virtualisation.qemu.console option in a module that's always imported
     virtualisation.qemu = {
       consoles = lib.optional (options ? virtualisation.qemu.consoles) qemuSerialDevice;
-      package  = pkgs.qemu_test;
+      package  = lib.mkDefault pkgs.qemu_test;
     };
 
     boot.initrd.preDeviceCommands =