[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