Install cargo-llvm-cov

So we can measure Rust test coverage on CI.

https://rustprojectprimer.com/measure/coverage.html

```
info: running `rustup component add llvm-tools-preview --toolchain 1.89.0-x86_64-apple-darwin` to install the `llvm-tools-preview` component for the selected toolchain
info: downloading component llvm-tools

```