gh-117649: Raise ImportError for unsupported modules in free-threaded build (#117651)

gh-117649: Raise ImportError for unsupported modules in free-threaded build (#117651)

The free-threaded build does not currently support the combination of
single-phase init modules and non-isolated subinterpreters. Ensure that
`check_multi_interp_extensions` is always `True` for subinterpreters in
the free-threaded build so that importing these modules raises an
`ImportError`.