Fix long Rust command lines on Windows with response files

Windows builds of large Rust crates such as once-core were failing because the rustc command line exceeded the Windows CreateProcess limit. Once now routes the entire post-rustc argument list through a single response file on Windows, leaving only the toolchain path and @response-file reference on the command line. This applies to both the build-script and main crate compiles, so Windows release and graph builds can succeed.