nixos/test-driver: fix GPU Qemu argument condition

In 48995a6ef9 (#527511), the `isLinux`
and `isAarch64` condition were taken from the `hostPkgs`, and not the
guest system's `pkgs`.

Graphical NixOS tests on aarch64-darwin work with this commit thanks
to the removed `isLinux` argument, and the `isAarch64` argument should
be taken from the guest's OS, per [qemu's docs][1]:

> The recommended option is virtio-gpu-pci; this is the only one which
> will work correctly with KVM. You may also need to ensure your guest
> kernel is configured with support for this;

[1]: https://www.qemu.org/docs/master/system/arm/virt

nixos/lib/testing/driver.nix

@@ -14,7 +14,7 @@ let
 
   inherit (config) sshBackdoor;
 
-  inherit (hostPkgs.stdenv.hostPlatform) isLinux isAarch64;
+  inherit (hostPkgs.stdenv.hostPlatform) isLinux;
 
   # Reifies and correctly wraps the python test driver for
   # the respective qemu version and with or without ocr support
@@ -249,7 +249,7 @@ in
     passthru.driver = config.driver;
 
     nodeDefaults =
-      { config, ... }:
+      { config, pkgs, ... }:
       {
         # This is needed for the SSH backdoor to function.
         # Set this to `true` by default to not change essential QEMU flags
@@ -258,7 +258,7 @@ in
         # If needed, this can still be turned off.
         virtualisation.qemu.enableSharedMemory = lib.mkDefault isLinux;
         # Needed for screenshots to work (in e.g `nixosTests.login`)
-        virtualisation.qemu.options = lib.optionals (isLinux && isAarch64) [
+        virtualisation.qemu.options = lib.optionals pkgs.stdenv.hostPlatform.isAarch64 [
           "-device virtio-gpu-pci"
         ];