Add Rust toolchain and wasi-sdk to flake.nix

[tomasol]

Description of the change

Fixes #1063

Checklist

• I've updated the default plugin import namespace and incremented the major version of javy-plugin-api if the QuickJS bytecode has changed.
• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli, javy-plugin, and javy-plugin-processing do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.

[saulecabrera]

Thanks for this. I think in my case everything was working as expected, regarding the WASI SDK, given that I have nix-ld configured. However, I do agree that this approach is cleaner.