Add sccache to Rust CI jobs

[ChristianPavilonis]

Summary

• add mozilla-actions/sccache-action@v0.0.10 to Rust CI jobs
• configure SCCACHE_GHA_ENABLED=on and RUSTC_WRAPPER=sccache for subsequent Cargo steps
• include the integration test environment composite action while leaving existing rust-cache setup in place

Validation

• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml", ".github/actions/**/*.yml"].each { |f| YAML.load_file(f); puts "ok #{f}" }'
• git diff --check
• grep -R "mozilla-actions/sccache-action" -n .github/workflows .github/actions
• grep -R "RUSTC_WRAPPER\|SCCACHE_GHA_ENABLED" -n .github/workflows .github/actions

Note: actionlint was not installed locally.