feat: introduce riscv64 to linux and musllinux builds

[eshattow]

Adding RISC-V to build and publish, depends on (#619 tracking issue) use of https://riseproject-dev.github.io/riscv-runner/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.68%. Comparing base (58b364c) to head (c2f74e9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
- Coverage   85.74%   85.68%   -0.06%     
==========================================
  Files          29       29              
  Lines        3487     3487              
  Branches      602      602              
==========================================
- Hits         2990     2988       -2     
- Misses        307      308       +1     
- Partials      190      191       +1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing eshattow:riscv64 (c2f74e9) with main (58b364c)}

[Copilot]

Pull request overview

Introduces a RISC-V (riscv64) build target into the wheel build workflow so Linux and musllinux wheels can be produced for that architecture.

Changes:

• Add ubuntu-24.04-riscv to the build_wheels job matrix.
• Configure CIBW_ARCHS_LINUX to select riscv64 when running on the RISC-V runner.

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[Copilot]

matrix.arch is referenced in the job name, but the matrix only defines os and musl, so the displayed job name will have a blank/null arch value. Consider either adding an explicit arch dimension to the matrix (and deriving runs-on/CIBW_ARCHS_LINUX from it), or removing with arch ${{ matrix.arch }} from the name to avoid confusion—especially now that multiple CPU architectures are being added.

[Copilot]

ubuntu-24.04-riscv is not a standard GitHub-hosted runner label, and this repo doesn’t appear to document or gate the requirement for a self-hosted RISC-V runner. Without a guard, CI will queue indefinitely/fail in forks or environments without that runner. Consider adding an if: condition (e.g., only run on the main repo) and/or documenting the expected runner labels in the workflow (or repository docs) so contributors understand the dependency.