Use stable Rust for cargo fmt checks in CI

[ianthetechie]

I assume this was set before the 2024 edition dropped, which cleaned up a lot of gripes some people had with rustfmt. Would you be OK with updating this to use the stable channel instead?

[kylebarron]

I really like import sorting, which isn't stable: https://rust-lang.github.io/rustfmt/?version=v1.8.0&search=#group_imports

[ianthetechie]

Fair enough ;) In that case:

1. Should we also add the nightly formatter requirement to the contributor docs?
2. Does it require a specific nightly version? Or can we just tell contributors to add +nightly?

[kylebarron]

1. Should we also add the nightly formatter requirement to the contributor docs?

Yes! We should! Would you like to make a PR for that?

3. Does it require a specific nightly version? Or can we just tell contributors to add +nightly?

I don't think so. I think any nightly is fine.

But I don't want spurious errors in CI so in CI I have it pinned to a specific date. But I'm fine with periodically updating that nightly date.