`build.rs` changes in rust 1.77

There is a new syntax for `build.rs` in Rust/cargo 1.77.

This entails the use of `metadata` and inclusion of an extra `:` everywhere.

Once we upgrade to 1.77, we have to be aware of this change, as the previous syntax would work,
but it won't receive updates internally.

See this commit as an example changes needed:

As of this writing, this is not an urgent issue, as we are barely on Rust 1.70 in extendr ( due to compliance challenges with CRAN ), however, it is better to document this now.