Thread cargo build script directives into downstream Rust actions

Cargo build script directives are now threaded into downstream granular Rust rustc actions. When you declare [[cargo.build_script]] in your TOML build graph, dependents consume the captured directives instead of falling back to the cargo.binary escape hatch. This gives build-script-backed crates proper action digests, restored directive output paths, and working cache hits and invalidation for cfg propagation and other build-script outputs.