pymongoarrow: add build-pymongoarrow.yml for riscv64 wheels

[luhenry]

• Package: pymongoarrow
• Version: 1.15.0
• Source: https://github.com/mongodb-labs/mongo-arrow/tree/main/bindings/python
• Docs: https://www.mongodb.org

Cython extension bridging BSON, PyArrow and NumPy, linked against a from-source libbson and the pyarrow wheel's own Arrow C++ libraries. Upstream publishes no riscv64 wheel.

Mirrors upstream's dist-python.yml and the checkout's own [tool.cibuildwheel] config, adapted for riscv64.

Differs from upstream

• riscv64 manylinux image, CMAKE_POLICY_VERSION_MINIMUM=3.5 - CMake 4 in the image rejects an old floor somewhere under libbson's tree
• CIBW_BEFORE_BUILD builds libbson directly - upstream's dev-group sync pulls in watchfiles, which has no riscv64 Rust target
• LIBBSON_INSTALL_DIR pinned absolute - before-build and the CMake configure run with different working directories
• PIP_EXTRA_INDEX_URL/scoped PIP_ONLY_BINARY - pyarrow/numpy/pymongo have no riscv64 wheel on PyPI

Testing

• no riscv64 mongod exists anywhere, so only the DB-independent unit tests run (bson/builders/schema/monkey/libbson) - the same constraint build-pymongo.yml documents for the driver

License: OK

Built on cp312; 58 passed, 0 skipped.