rxvt-unicode: set `meta.mainProgram` to "urxvt" (#397723)

pkgs/applications/terminal-emulators/rxvt-unicode/default.nix

···
     maintainers = with maintainers; [ rnhmjoj ];
     platforms = platforms.unix;
     license = licenses.gpl3;
+    mainProgram = "urxvt";
   };
 }