build.rs captures the absolute path to cc/gcc/clang during compilation
and bakes it into the binary. On Nix systems this embeds the full
/nix/store path so `lux compile` works without cc on PATH.
Lookup order: $CC env var > embedded build-time path > PATH search.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
