- Replace cross-rs with Zig toolchain and cargo-zigbuild
- Add rustup target installation step for all platforms
- Include --target flag in native Linux build for consistent output paths
cross-rs has no Docker image for macOS targets and the Windows image
ships with a broken cargo PATH, causing all non-Linux builds to fail.
Zig provides its own cross-linker and ships macOS/Win libc headers,
enabling cross-compilation for all three targets from a single Linux
runner.