Revert "nixos/test-instrumentation: softcode DISPLAY"

This reverts commit 4531ec5955.

This broke waitForX: https://github.com/NixOS/nixpkgs/commit/b11ac5d29dcccdb7474a23ac1d8a6c40553adcd5#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;
         }
     });