nixos/opentabletdriver: fix "Daemon is already running" (#473683)

nixos/modules/hardware/opentabletdriver.nix

@@ -8,7 +8,10 @@ let
   cfg = config.hardware.opentabletdriver;
 in
 {
-  meta.maintainers = with lib.maintainers; [ thiagokokada ];
+  meta.maintainers = with lib.maintainers; [
+    gepbird
+    thiagokokada
+  ];
 
   options = {
     hardware.opentabletdriver = {
@@ -60,6 +63,12 @@ in
 
       serviceConfig = {
         Type = "simple";
+        # workaround for https://github.com/NixOS/nixpkgs/issues/469340
+        ExecStartPre = pkgs.writeShellScript "disable-for-gdm-greeter" ''
+          if [[ "$USER" = "gdm-greeter"* ]]; then
+            exit 1
+          fi
+        '';
         ExecStart = lib.getExe' cfg.package "otd-daemon";
         Restart = "on-failure";
       };