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/installer/tools/nixos-build-vms/build-vms.nix

@@ -6,7 +6,12 @@
 let
   nodes = builtins.mapAttrs (vm: module: {
     _file = "${networkExpr}@node-${vm}";
-    imports = [ module ];
+    imports = [
+      module
+      ({ pkgs, ... }: {
+        virtualisation.qemu.package = pkgs.qemu;
+      })
+    ];
   }) (import networkExpr);
 in