ci: add riscv64 target to linux wheel build matrix

[gounthar]

Summary

Add riscv64 to the maturin-action linux build matrix in python-release.yml so that linux_riscv64 wheels are built and published to PyPI.

maturin-action cross-compiles via riscv64gc-unknown-linux-gnu cargo target — no QEMU or additional infrastructure needed.

Evidence

• Tested wheel: tokenizers-0.22.2-cp39-abi3-manylinux_2_34_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz)
• Build time: ~20 min
• Imports and passes basic smoke test on riscv64

Context

• riscv64gc-unknown-linux-gnu is a Rust tier-2 target with std available
• maturin-action handles cross-compilation transparently
• Sibling HuggingFace project safetensors has a similar PR open
• RISC-V hardware is shipping (SiFive, SpacemiT, Sophgo SG2044)

Fixes #1961

[gounthar]

Closing in favor of #1951 by @threexc (BayLibre/RISE) which was opened first. Coordinating to avoid duplicates going forward.