[Chronik] Format Rust code with the latest nightly + update CI
Summary:
Previously, let-else wasn't formatted at all, because the formatter didn't support it.
Users trying to push code with the latest formatter will have arcanist complain, so we format the code and update the CI to the latest nightly version.
Test Plan: arc lint
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D15047