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: jiter-0.13.0-cp313-cp313-manylinux_2_34_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz)
• Build time: ~5 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 project pydantic-core already has riscv64 support merged (pydantic/pydantic#12723)

Fixes #236

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[codspeed-hq]

Merging this PR will not alter performance

✅ 70 untouched benchmarks

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_{Comparing gounthar:feat/add-riscv64-wheel (03ddad8) with main (312d268)}