waybar: Avoid reload onChange when systemd integration is enabled

When systemd integration is enabled, systemd will handle the issuing of
`SIGUSR2` through `X-Reload-Triggers`.

All this would do is trigger multiple `SIGUSR2` signals, which can
exacerbate this upstream issue:
https://github.com/Alexays/Waybar/issues/3344#issuecomment-2390661508

modules/programs/waybar.nix

@@ -318,7 +318,7 @@ in
 
         xdg.configFile."waybar/config" = mkIf (settings != [ ]) {
           source = configSource;
-          onChange = ''
+          onChange = mkIf (!cfg.systemd.enable) ''
             ${pkgs.procps}/bin/pkill -u $USER -USR2 waybar || true
           '';
         };
@@ -329,7 +329,7 @@ in
               cfg.style
             else
               pkgs.writeText "waybar/style.css" cfg.style;
-          onChange = ''
+          onChange = mkIf (!cfg.systemd.enable) ''
             ${pkgs.procps}/bin/pkill -u $USER -USR2 waybar || true
           '';
         };