fix: keep Batocera's save files apart from bios/

batocera-systems lists bios/saves/dolphin-emu/Triforce/segaboot.gcm, so the pack folder for the save directory is userdata-saves/. validate_schemas refuses a pack folder the BIOS tree already uses.

scripts/packextras.py

@@ -1371,7 +1371,7 @@ def _save_fields(extra: dict, pack_path: str, placement: ExtraPlacement) -> None
     extra["pack_path"] = pack_path
     if placement.save is not None:
         # The path below the save directory, what the manifest writes.
-        extra["save_dest"] = pack_path[len(placement.save.folder) + 1:]
+        extra["save_dest"] = pack_path[len(placement.save.in_pack) + 1:]
         if not placement.save.by_core:
             extra.pop("core_folder", None)