Use target compilation mode for Cargo build scripts

[dzbarsky]

cargo_build_script now applies config.exec().and_then(...) only to the script attribute and restores the parent compilation_mode through target_compilation_mode. The runner no longer has a second deps exec edge; it reads transitive BuildInfo values from the script binary’s DepInfo.

The new rolling-Bazel aquery regression compares the target and build-script dependency Rustc arguments, environment, execution requirements, execution platform, mapped inputs and outputs, param files, and process_wrapper. Validation passed on Bazel main and the Bazel 9.1.1 fallback; a remote-cache build reported one deduplicated spawn with matching SHA-256 digests.