CI: Use cargo 1.84 feature to check MSRV

[msrd0]

https://blog.rust-lang.org/2025/01/09/Rust-1.84.0.html#cargo-considers-rust-versions-for-dependency-version-selection

Our MSRV is currently 1.65, otherwise we cannot find a candidate for the regex-syntax crate which is a transitive dependency.