Keep the numba cache test type-checking on any numba version (#5512)

numba is an unpinned transitive dependency and only its newer releases ship type information. Where mypy can read numba it sees CACHE_DIR being assigned at runtime and rejects the lookup; on older versions it never looks at all, so the ignore the first case needs is reported as unused in the second and broke every local commit. Mark it unused-ignore as well, as done elsewhere in the codebase.