Include --check-cfg=cfg(kani) in the rust flags to avoid a warning about an unknown cfg.

[zhassan-aws]

Starting with the 2024-05-05 toolchain (and the upcoming Rust 1.80 release), the unexpected_cfgs lint has been turned on by default. As a result, running cargo kani on a crate that has a #[cfg(kani)] results in a warning (see #3186). To avoid this warning, this PR adds --check-cfg=cfg(kani) to RUSTFLAGS when Kani invokes cargo.

Call-outs: On such packages, doing a cargo build will also result in this warning, unless:

println!("cargo::rustc-check-cfg=cfg(kani)");

is added to the package's build.rs file. However, this warning would only occur with cargo build if the package uses the 2024-05-05 toolchain (or newer), or the Rust version used in the package is upgraded to 1.80 (when it's released at the end of July 2024). Since we're likely to release a new version of Kani sooner than the 1.80 release, this PR mitigates the issue that is more likely to impact users (a warning from cargo kani).

Resolves #3186

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

Thank you for following up on this one!