river-tag-overlay: unmark broken on aarch64

This package didn't build on aarch64 when it was added, but now it
does. In the meantime we bumped the default GCC on aarch64, so
presumably that fixed it.

pkgs/applications/misc/river-tag-overlay/default.nix

···
     license = licenses.gpl3Only;
     maintainers = with maintainers; [ edrex ];
     platforms = platforms.linux;
-    broken = stdenv.isAarch64;
   };
 }