opt: set XDG_CURRENT_DESKTOP var in code not in desktop file

maomao.desktop

@@ -2,6 +2,6 @@
 Encoding=UTF-8
 Name=Maomao
 Comment=maomao WM
-Exec=env XDG_CURRENT_DESKTOP=maomao maomao
+Exec=maomao
 Icon=maomao
 Type=Application

src/maomao.c

@@ -4930,6 +4930,8 @@ run(char *startup_cmd) {
   if (!wlr_backend_start(backend))
     die("startup: backend_start");
 
+  setenv("XDG_CURRENT_DESKTOP", "maomao", 1);
+
   /* Now that the socket exists and the backend is started, run the startup
    * command */
   if (startup_cmd) {