feat: add riscv64 manylinux wheels to build matrix

[gounthar]

Summary

Add linux_riscv64 manylinux wheels to the release build matrix.

Changes

• Add manylinux-riscv64 entry to the wheel job matrix
• Add QEMU setup step for riscv64 emulation on x86_64 runners

Evidence

• Tested wheel: pyzmq-26.3.0-cp313-cp313-linux_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz, 16 GB RAM)
• Imports and passes basic smoke test on riscv64

Context

• manylinux_2_28_riscv64 is available in pypa/manylinux
• cibuildwheel 3.x supports riscv64 via QEMU
• Several packages already ship riscv64 wheels: aiohttp, yarl, multidict, propcache
• RISC-V hardware is shipping (SiFive, SpacemiT, Sophgo SG2044)

closes #2163

[minrk]

Thanks for the PR! This doesn't appear to add any wheels, though.

[gounthar]

@minrk You're right 🤦 I added the QEMU setup step but forgot the matrix entry itself. Fixed in de13bcb: added the manylinux-riscv64 entry with arch: riscv64 and build: "*manylinux*".

Sorry about that.

[minrk]

Thanks! This does take many times longer than any other platform to just build one wheel, so I might need to rethink how it's done a bit (most of the time is spent rebuilding libzmq/libsodium dependencies which are very infrequently released and could come from binaries stashed somewhere...

[gounthar]

@minrk Thanks for looking at this! Caching the libzmq/libsodium builds makes a lot of sense; they rarely change, and rebuilding under QEMU is the slowest part. Happy to help test any caching approach you come up with.