copyq: support xwayland

We do this by adding QT_QPA_PLATFORM=xcb to the service environment as
per the workaround given in

  https://copyq.readthedocs.io/en/latest/known-issues.html#known-issue-wayland

Fixes #3530

modules/services/copyq.nix

@@ -46,6 +46,7 @@ in {
       Service = {
         ExecStart = "${cfg.package}/bin/copyq";
         Restart = "on-failure";
+        Environment = [ "QT_QPA_PLATFORM=xcb" ];
       };
 
       Install = { WantedBy = [ cfg.systemdTarget ]; };