pikepdf: add build-pikepdf.yml for riscv64 wheels

[luhenry]

• Package: pikepdf
• Version: 10.12.0
• Source: https://github.com/pikepdf/pikepdf
• Docs: https://pikepdf.readthedocs.io/

Builds qpdf 12.3.2 from source (CMake, native crypto provider) then compiles pikepdf's nanobind/qpdf C++ extension against it. Upstream publishes no riscv64 wheel.

Mirrors upstream's build.yml (wheels_linux job).

Differs from upstream

• Adds CIBW_MANYLINUX_RISCV64_IMAGE - upstream has no riscv64 image configured.
• Repeats QPDF_MIN_VERSION/QPDF_VERSION/QPDF_PATTERN in CIBW_ENVIRONMENT - the env var replaces, not merges, upstream's [tool.cibuildwheel.environment] table.
• Drops musllinux - no musllinux_riscv64 manylinux image exists.

Testing

• same as upstream

License: OK

Built on cp312; wheel imports and the wheel's own pytest -nauto tests suite passes.

[luhenry]

Reopening as a non-draft PR (opened as draft by mistake).