[Chronik] Add line to CONTRIBUTING.md and install-dependencies.sh ensuring rustfmt when installing nightly

Summary:
On some machines, the current set of commands won't necessarily install the rustfmt component.

Adding this call to both CONTRIBUTING.md and install-dependencies.sh doesn't error when rustfmt is already there, but will install it if it isn't.

Test Plan: Run the commands in e.g. a clean docker environment, will either install rustfmt or output info: component 'rustfmt' for target '<target>' is up to date

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11718