Recent upstream changes added new options (`--build-bindgen`, `--build-vet`, `--build-crubit`) to `build_rust.py`. This makes a plain `--build` ambiguous under `argparse`, which breaks `build_rust_toolchain_aux.py` when it tries to forward `x.py` arguments. Fix by inserting `--` after `--run-xpy` so that `argparse` stops parsing and passes `--build` through as intended.