The build environment [already uses Ubuntu 20.04](https://github.com/rust-lang/crates-build-env/commit/acdd0f7caf930a6fda72873f6b401f68eba74f4a) but the docs [still reference Ubuntu 18.01](https://github.com/rust-lang/rust-forge/search?q=%2218.04%22&type=code)