In our GitHub Actions CI, update the Code Coverage step for the `libmarlin` crate to invoke Tarpaulin using the nightly toolchain. This ensures we pick up the latest coverage features and avoid the unexpected argument error:
* Replace the two-line `cargo tarpaulin` invocation with a single command using `cargo +nightly tarpaulin --package libmarlin --out Xml --fail-under 85`.
* Removes the extraneous line break and aligns with the rest of our Rust commands under nightly when needed.