Make sure that cargo-nextest is cached as well

[elasticdog]

The order matters here since rust-cache cleans out any files in ~/.cargo/bin that were present before the action ran, which means we'll want to install nextest after running the cache job. The install-action job will check for newer versions appropriately.

The one thing I'm not sure about is if rust-cache will key based off of the contents of the ~/.cargo/bin directory, or if it's just saving that into the cache. If it does key off of the contents, we may need to always install nextest even if it's not going to be used (for check/ format/lint jobs) so we can maintain cache efficiency.

See:

• https://github.com/Swatinem/rust-cache#cache-details
• Merge cache-cargo-install-action taiki-e/install-action#66

[elasticdog]

Note that the contents of ~/.cargo/bin are not part of the cache key, so adding tools won't automatically get picked up in the cache unless we manually remove the old caches (or the main branch cache's haven't been hit for a week), but that also means that we don't have to install all of the tools as part of every job that uses Rust. Maybe we should tweak the scheduled job to only preload the cache every 8 days, so we don't accidentally keep an old one warm.