libraspberrypi provided an empty directory. The during
https://github.com/NixOS/nixpkgs/pull/107637, this was refactored to use
cmakeFlags.
cmakeFlags can't use `$out` directly. `$out` is a bash variable, so to
use it there, `${placeholder "out"}` is needed, otherwise it'll
pick `"$out"` literally.
ce709163f2