ci: stabilize dwaar rust builds

[gopherine]

Summary

• Limit Rust CI build concurrency and disable incremental compilation for reliable small-runner builds.
• Apply the same Rust CI environment in GitHub Actions and Permanu CI.
• Fix the clippy doc-comment issue in self-update signing verification.

Test Plan

• Docker sandbox Dwaar Rust CI lane passed before rebase.
• Post-rebase diff check passed.
• scripts/permanu-ci-rust-env.sh passes bash -n.