Revert "nixos/test-instrumentation: softcode DISPLAY"

This reverts commit 4531ec5955.

This broke waitForX: b11ac5d29d (commitcomment-25486347)

cc @Lassulus

nixos/lib/test-driver/Machine.pm

@@ -602,11 +602,8 @@ sub waitForX {
     $self->nest("waiting for the X11 server", sub {
         retry sub {
             my ($status, $out) = $self->execute("journalctl -b SYSLOG_IDENTIFIER=systemd | grep 'session opened'");
-            my $display = $ENV{'DISPLAY'};
-            $display =~ s/://;
-
             return 0 if $status != 0;
-            ($status, $out) = $self->execute("[ -e /tmp/.X11-unix/X$display ]");
+            ($status, $out) = $self->execute("[ -e /tmp/.X11-unix/X0 ]");
             return 1 if $status == 0;
         }
     });

nixos/modules/testing/test-instrumentation.nix

@@ -5,10 +5,9 @@
 
 with lib;
 
-let
-  display = (d: if d == null then 0 else d) config.services.xserver.display;
-  kernel = config.boot.kernelPackages.kernel;
-in {
+let kernel = config.boot.kernelPackages.kernel; in
+
+{
 
   # This option is a dummy that if used in conjunction with
   # modules/virtualisation/qemu-vm.nix gets merged with the same option defined
@@ -29,7 +28,7 @@ in {
           ''
             export USER=root
             export HOME=/root
-            export DISPLAY=:${toString display}
+            export DISPLAY=:0.0
 
             source /etc/profile