Update Rust toolchain

Generics in latests nightlies with `generic_const_args` require `const {}` rather than `{}` in places like this:
```rust
atomic_i_increment::<_, { Scope::QueueFamily as u32 }, { Semantics::NONE.bits() }>(..)
```

This makes writing code that compiles both with `rust-gpu`'s old toolchain version and latest nightly very frustrating.

Please consider updating toolchain again some time soon 🙏