libavif: disable gdk-pixbuf thumbnailer when cross-compiling

gdk-pixbuf doesn't support thumbnailers when cross-compiling, causing
libavif to fail to cross-compile because it tries to wrap
${gdk-pixbuf}/bin/gdk-pixbuf-thumbnailer, which doesn't exist.

pkgs/development/libraries/libavif/default.nix

@@ -68,6 +68,9 @@ stdenv.mkDerivation rec {
     GDK_PIXBUF_MODULE_FILE=${gdkPixbufModuleFile} \
     gdk-pixbuf-query-loaders --update-cache
 
+  ''
+  # Cross-compiled gdk-pixbuf doesn't support thumbnailers
+  + lib.optionalString (stdenv.hostPlatform == stdenv.buildPlatform) ''
     mkdir -p "$out/bin"
     makeWrapper ${gdk-pixbuf}/bin/gdk-pixbuf-thumbnailer "$out/libexec/gdk-pixbuf-thumbnailer-avif" \
       --set GDK_PIXBUF_MODULE_FILE ${gdkPixbufModuleFile}