nixos-test-driver: unbreak eval on macOS (#512733)

nixos/lib/test-driver/default.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  stdenv,
 
   buildPythonApplication,
   colorama,
@@ -59,6 +60,8 @@ buildPythonApplication {
     socat
     util-linux
     vde2
+  ]
+  ++ lib.optionals stdenv.isLinux [
     vhost-device-vsock
   ]
   ++ lib.optionals enableNspawn [

nixos/lib/testing/driver-configuration.nix

@@ -1,7 +1,7 @@
 {
   config,
   lib,
-  pkgs,
+  hostPkgs,
   ...
 }:
 let
@@ -75,9 +75,9 @@ in
         ) (lib.attrValues config.nodes ++ lib.attrValues config.containers)
       );
       global_timeout = config.globalTimeout;
-      test_script = pkgs.writeText "test-script" config.testScriptString;
+      test_script = hostPkgs.writeText "test-script" config.testScriptString;
       enable_ssh_backdoor = config.sshBackdoor.enable;
     };
-    driverConfigurationFile = pkgs.writers.writeJSON "driverConfiguration.json" config.driverConfiguration;
+    driverConfigurationFile = hostPkgs.writers.writeJSON "driverConfiguration.json" config.driverConfiguration;
   };
 }

nixos/lib/testing/driver.nix

@@ -9,6 +9,8 @@ let
 
   inherit (config) sshBackdoor;
 
+  inherit (hostPkgs.stdenv.hostPlatform) isLinux;
+
   # Reifies and correctly wraps the python test driver for
   # the respective qemu version and with or without ocr support
   testDriver = config.pythonTestDriverPackage.override {
@@ -251,7 +253,7 @@ in
         # depending on whether debugging is enabled.
         #
         # If needed, this can still be turned off.
-        virtualisation.qemu.enableSharedMemory = lib.mkDefault true;
+        virtualisation.qemu.enableSharedMemory = lib.mkDefault isLinux;
 
         assertions = [
           {