style: deny(unexpected_cfgs)

[SteveLauC]

What does this PR do

The default level for this lint is warn, let's deny it. Though I am not sure if this is only available since Rust 1.80, let's verify this in our CI.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Though I am not sure if this is only available since Rust 1.80, let's verify this in our CI.

Ok, I get it now, we should get a Clippy warning of unknown lint, but this is allowed in our CI:

cargo clippy --target aarch64-apple-darwin --all-targets --all-features -- -D warnings -A unknown-lints