* ci: speed up Rust CI builds with caching and prebuilt tools
The Windows builder takes ~6min vs ~3min on Ubuntu, largely because
cargo-llvm-cov is compiled from source on every run. This adds
Swatinem/rust-cache for build artifact caching and switches to
taiki-e/install-action for prebuilt cargo-llvm-cov binaries.
Also fixes the undefined `inputs.shell` references by hardcoding `bash`.
* fix: add Swatinem and taiki to cspell dictionary
* ci: remove rust-cache, no measurable benefit for this project
Investigated cache hit/miss across 3 CI runs. Even with warm cache
hits (macOS/Windows), build times were unchanged or slightly slower
due to cache download/extract overhead (~39MB) outweighing the small
dependency compile time savings.
Eric Seidel
0af491b628