Regenerate index if `StoreIdentityCids` is enabled and existing index does not have `FullyIndexed` characteristic

[masih]

ReadOrGenerateIndex does not re-generate index if the existing index doesn't satisfy passed options.

https://github.com/ipld/go-car/blob/f4378127a67f1bf4d616ef83a1e5a4d841a9478b/v2/index_gen.go#L146-L149