Passing custom flags to `cargo`

I'm currently trying to get [`tokenizers`](https://github.com/huggingface/tokenizers/) working in Pyodide, and this requires using the `unstable_wasm` feature. If I understand correctly, this means that I need to pass `--features unstable_wasm` to cargo, which would require something like `built.cargoflags` in `meta.yaml`? Or is there some other way to achieve this that could suffice for now?