Fix Windows Rust builds that hit the command-line length limit

Windows release builds of crates with large dependency graphs such as once-core previously failed with failed to spawn rustc.exe: The filename or extension is too long. (os error 206) because the Windows 32 KB command-line limit was being exceeded by --extern and -L flags. The full post-rustc argument list for both the build-script compile and the main crate compile is now routed through a single response file on Windows, so only the toolchain path and @response-file reference remain on the command line. Non-Windows hosts keep the inline invocation. Action caching stays correct because the response file contents are folded into the action digest.