eos-installer: set meta.mainProgram

pkgs/applications/misc/eos-installer/default.nix

···
     description = "Installer UI which writes images to disk";
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ qyliss ];
+    mainProgram = "gnome-image-installer";
     platforms = platforms.linux;
   };
 }