cargo_build_script with local strategy re-serializes CC toolchain flags without the toolchain inputs

With `--strategy=CargoBuildScriptRun=local`, `cargo_build_script` passes the CC toolchain flags to `cc-rs`, which runs with a CWD outside the sandbox. The toolchain inputs are not propagated, so a flag that points at a toolchain file does not resolve.

We hit it with a force included header. The flag was `-imacros external/toolchains_llvm++llvm+llvm_toolchain/redacted_dates.h` and build scripts failed with: `fatal error: 'external/.../redacted_dates.h' file not found`

The reproducer is in the linked issue. Any target pulling a `*-sys` crate with a C build script, plus `--strategy=CargoBuildScriptRun=local`. Reported on bazel 7.5.0, rules_rust 0.70.0, on linux-x86_64 and darwin-aarch64. We did not reduce it to a rules_rust only case.

We reverted our side, so it is not urgent for us anymore. The gap stays though. `cargo_build_script_runner` does not propagate `cxx_builtin_include_files` to local strategy invocations, so the same happens with any toolchain flag that references a file.

There is a second half. Flag values that need quotes, for example `-D__DATE__="redacted"`, may lose the quotes on the way into the build script. Clang then sees a bare token and errors. We did not verify that part ourselves.

Start by reading cargo_build_script_runner and the local CargoBuildScriptRun strategy path, then inspect the linked reproducer. Trace how cxx_builtin_include_files and quoted flag values are passed into local invocations. Done means toolchain-referenced flags resolve outside the sandbox and the reported quoting case is verified or clearly scoped.

Written by the indexing model from the issue text.