NixOS VM tests: Don't create a setgid group in vde_switch

Nix no longer allows this for security reasons.

http://hydra.nixos.org/build/53993125

nixos/lib/test-driver/test-driver.pl

···
     if ($pid == 0) {
         dup2(fileno($pty->slave), 0);
         dup2(fileno($stdoutW), 1);
-        exec "vde_switch -s $socket" or _exit(1);
+        exec "vde_switch -s $socket --dirmode 0700" or _exit(1);
     }
     close $stdoutW;
     print $pty "version\n";

pkgs/build-support/vm/windows/controller/default.nix

···
     MONITOR_SOCKET="$(pwd)/monitor"
     WINVM_PIDFILE="$(pwd)/winvm.pid"
     CTRLVM_PIDFILE="$(pwd)/ctrlvm.pid"
-    ${vde2}/bin/vde_switch -s "$QEMU_VDE_SOCKET" &
+    ${vde2}/bin/vde_switch -s "$QEMU_VDE_SOCKET" --dirmode 0700 &
     echo 'alive?' | ${socat}/bin/socat - \
       UNIX-CONNECT:$QEMU_VDE_SOCKET/ctl,retry=20
   '';