The challenge-bypass-ristretto-ffi Rust crate is tricky to build

[Jean-Paul Calderone]

The crate2nix generated expression for the Rust crate points at the local filesystem for its source. However, the source is only present if git submodules have been properly updated and it is easy to forget to do this, resulting in a weird build error. Additionally, the resulting build then depends on what version you had checked out (maybe with changes), making the results less predictable

Make this crate point to the Github for its source.