zxing-cpp: add build-zxing-cpp.yml for riscv64 wheels

[luhenry]

• Package: zxing-cpp
• Version: 3.1.1
• Source: https://github.com/zxing-cpp/zxing-cpp
• Docs: https://github.com/zxing-cpp/zxing-cpp

Compiles the ZXing-C++ barcode/QR-code reading and writing library into a nanobind Python extension. Upstream publishes no riscv64 wheel.

Mirrors upstream's publish-python.yml, building wrappers/python straight from the checkout.

Differs from upstream

• Drops cp310/cp311 and musllinux - matches this repo's cp312 floor and upstream's own skip list.

Matrix: cp312 (abi3, also covers cp313/cp314) and cp314t only - the nanobind module is built STABLE_ABI FREE_THREADED, so one non-free-threaded build satisfies every interpreter.

Testing

• same as upstream

License: OK

Built on cp312; pending CI results.