build.rs changes in rust 1.77

[CGMossa]

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: eb5e95e

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.