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

[luhenry]

• Package: awkward-cpp
• Version: 56
• Source: https://github.com/scikit-hep/awkward-1.0 (monorepo, awkward-cpp/)
• Docs: https://awkward-array.org

Compiles Awkward Array's pybind11/scikit-build-core C++ kernel extension, the compiled backend the pure-Python awkward package depends on. Upstream publishes no riscv64 wheel.

Mirrors upstream's make_sdist/build_wheels jobs.

Differs from upstream

• Pins a commit, not a tag - awkward-cpp's release job never pushes one; verified byte-identical to the PyPI sdist.
• Drops {project}/tests from the test command - that's the parent awkward package's suite, out of scope for this port.
• CIBW_BUILD_FRONTEND: build, not upstream's build[uv] - the audit step needs a host uv binary the self-hosted runner lacks.
• Builds manylinux only - no numpy riscv64 wheel exists for musllinux anywhere.

Testing

• Same kernel test suite upstream runs (tests-cpu-kernels[-explicit], tests-spec[-explicit]).

License: OK

Built on cp312 equivalent (macOS arm64, local, from the released sdist); 22398 passed, 672 skipped.