libraspberrypi: fix output

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.

pkgs/development/libraries/libraspberrypi/default.nix

@@ -27,7 +27,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake pkg-config ];
   cmakeFlags = [
     (if (stdenv.hostPlatform.isAarch64) then "-DARM64=ON" else "-DARM64=OFF")
-    "-DVMCS_INSTALL_PREFIX=$out"
+    "-DVMCS_INSTALL_PREFIX=${placeholder "out"}"
   ];
 
   meta = with stdenv.lib; {