cargo: apply explicit lint config to build scripts

[jrandolf]

Allow cargo_build_script to accept an explicit lint_config and forward it to build-script compilation. Explicit rustc lints also apply in execution configurations, so build scripts and procedural macros honor the configuration attached to them.

Analysis coverage checks the configured rustc lint and check-cfg flags on a library, a build script, and a proc-macro in both target and execution configurations. Existing Clippy and rustdoc cases remain in the suite.

Validation: all five tests and the real fixture builds pass locally on macOS arm64 with Bazel 9.2.0 at bfd2845 (bazel test //test/unit/lint_flags:all). The new proc-macro case checks both target and execution configurations. Formatting checks also pass.

Closes #3486.

[UebelAndre]

Thank you! Could you also add a test that shows lint_config applying to a proc-macro (unless we already have a test for that then it's fine)

[jrandolf]

Added proc_macro_lints_apply_in_exec in 2774489. It attaches the explicit lint configuration to a rust_proc_macro and checks both --allow=unknown_lints and --check-cfg=cfg(bazel) in the target and execution configurations. The assertions also require a Rustc action so the test cannot pass by finding no compilation to inspect.

All five tests and the real fixture builds now pass locally on macOS arm64 with Bazel 9.2.0 at 2774489.