nix: bump scenefx flake and fix build

flake.nix

@@ -25,12 +25,13 @@
 
       perSystem = {
         config,
+        system,
         pkgs,
         ...
       }: let
-        inherit (pkgs) callPackage ;
+        inherit (pkgs) callPackage;
         mango = callPackage ./nix {
-          inherit (inputs.scenefx.packages.${pkgs.stdenv.hostPlatform.system}) scenefx;
+          scenefx = inputs.scenefx.packages.${system}.default;
         };
         shellOverride = old: {
           nativeBuildInputs = old.nativeBuildInputs ++ [];