While updating the Rust version I made a few other changes:

• The engine isn't building with Rust 1.80.0 (at least not the revision of the engine pinned in this branch). So I changed the engine build expression to use the Rust toolchain configuration from that project's rust-toolchain.toml file.
• I noticed a warning that was recently added to Crane (a Nix builder for Rust projects), and I made the recommended change.
• Fixed an error generated by a new lint check.
• Added some notes on how to update the Rust version in the future, and how to update things generally.