[Rust] Heads up -- change to the Rust CI system

[Andrew Lamb]

I just wanted to give everyone a heads up that we have merged an overhaul
of the Rust CI system.

You can see more details and rationale on the PR:
https://github.com/apache/arrow/pull/8821 but I wanted to bring it to the
attention of a wider audience as the PR checks will look different for Rust
PRs.

Thanks to @Jorge Cardoso Leitão <jorgecarlei...@gmail.com>  for driving
this work

Andrew