Improve CI workflow with rust-cache, clippy flags, and local RUSTFLAGS

[z80020100]

Summary

• Add Swatinem/rust-cache@v2 to all three CI jobs for faster builds
• Add Update toolchain step to test job for consistency with other jobs
• Change cargo clippy to cargo clippy --all-targets to lint tests, benches, and examples
• Move RUSTFLAGS: -Dwarnings from CI env to .cargo/config.toml so warnings are errors everywhere (local dev, CI, pre-commit hook)
• Fix envsetup.sh invocation from make setup && source to bash

Test plan

• cargo fmt --all -- --check passes
• cargo clippy --all-targets passes
• cargo test passes
• CI workflow runs successfully on this PR