cgminer: respect xserver.display variable

nixos/modules/services/misc/cgminer.nix

···
 
       environment = {
         LD_LIBRARY_PATH = ''/run/opengl-driver/lib:/run/opengl-driver-32/lib'';
-        DISPLAY = ":0";
+        DISPLAY = ":${toString config.services.xserver.display}";
         GPU_MAX_ALLOC_PERCENT = "100";
         GPU_USE_SYNC_OBJECTS = "1";
       };