chore: Add lint exception to make CI pass again

[umgefahren]

In Rust 1.80 cfg checking was introduced in stable. Since we use cfg(nightly) in one test to check the number of variants, we need to tell the compiler that this is okay.

It might seem like a dirty fix, but it's a widely used workaround:

• cfg(nightly) exception in Cargo.toml
• cfg(nightly) in Rust files

It's also one of the suggested solutions by rust itself (and it's more efficient then using a build.rs file).

Originally posted by @umgefahren in https://github.com/multiformats/rust-multiaddr/issues/110#issuecomment-2265472969