ci: add riscv64 target to linux wheel build matrix

[gounthar]

Summary

Add riscv64 to the maturin-action linux build matrix 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: openai_harmony-0.0.8-cp38-abi3-manylinux_2_34_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz)
• Build time: ~3 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
• RISC-V hardware is shipping (SiFive, SpacemiT, Sophgo SG2044)

Fixes #102

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Note: this work is part of the RISE Project effort to improve Python ecosystem support on riscv64 platforms. Native riscv64 CI runners are available for free via RISE RISC-V runners.

[gounthar]

Native build verified on riscv64 hardware (BananaPi F3, SpacemiT K1, rv64gc, GCC 14.2.0): SUCCESS (from git clone). Note: building from PyPI sdist with --no-binary :all: fails due to a ../README.md path issue; reported in #105.

[threexc]

@gounthar I've got one open for this as well already: #98

[gounthar]

Closing in favor of #98 by @threexc (BayLibre/RISE) which was opened first and is more thorough. Coordinating to avoid duplicates going forward.