Add riscv64 Python wheel builds

[gounthar]

Feature request

Add linux/riscv64 to the wheel packaging pipeline so that pre-built DuckDB Python wheels are available on PyPI for RISC-V 64-bit users.

Evidence

DuckDB Python wheel builds successfully on native riscv64 hardware:

• Wheel: duckdb-1.5.0-cp313-cp313-linux_riscv64.whl (20MB)
• Hardware: BananaPi F3 (SpacemiT K1, rv64gc, GCC 14.2.0)
• Query test: SELECT 'Hello from RISC-V!' AS message; → works

Proposed change

Add { os: ubuntu-24.04-riscv, arch: riscv64, cibw_system: manylinux } to the wheel build matrix in packaging_wheels.yml. Uses native RISE riscv64 runners (free for open source via https://github.com/apps/rise-risc-v-runners).

Related: C++ CLI release PR duckdb/duckdb#21496

Note: this work is part of the RISE Project effort to improve Python ecosystem support on riscv64 platforms.