Since Rust on macOS is slow, don't wait for the macOS Rust job to finish.
Instead, split rust into slow rust (macOS) and fast rust (Linux). And
allow the build to finish before slow rust finishes.

Also make sure that we have:
* a Rust build on each platform,
* a Rust build with each compiler, and
* a check on all our Rust builds.

Finally, sort builds: allow fail last, macOS first, slowest first.

Closes 31859 for 0.3.5.