ut1999: fix library search paths on x86_64-linux

Without the change the game fails to start as it fails to find it's
shared objects as:

    $ NIXPKGS_ALLOW_UNFREE=1 nix build -f. ut1999 && ./result/bin/ut1999
    Object not found: Class Render.Render
    History: UObject::SafeLoadError <- UObject::StaticLoadClass <- UGameEngine::Init <- InitEngine <- main
    Exiting due to error

After the change the game starts without problems.

pkgs/by-name/ut/ut1999/package.nix

@@ -102,8 +102,9 @@ in stdenv.mkDerivation {
     runHook postInstall
   '';
 
-  # .so files in the SystemARM64 directory are not loaded properly on aarch64-linux
-  appendRunpaths = lib.optionals (stdenv.hostPlatform.system == "aarch64-linux") [
+  # Bring in game's .so files into lookup. Otherwise game fails to start
+  # as: `Object not found: Class Render.Render`
+  appendRunpaths = [
     "${placeholder "out"}/${systemDir}"
   ];