rcssmin: add build-rcssmin.yml for riscv64 wheels

[luhenry]

• Package: rcssmin
• Version: 1.2.2
• Source: https://github.com/ndparker/rcssmin
• Docs: https://opensource.perlig.de/rcssmin/

Compiles the _rcssmin C extension, a CSS minifier with a pure-Python fallback. Upstream publishes no riscv64 wheel and carries no GitHub Actions workflow to mirror; this follows the same shape as the repo's other tiny build-from-checkout ports (e.g. build-editdistance.yml).

Differs from upstream

• CIBW_ENVIRONMENT: SETUP_CEXT_REQUIRED=1 - setup.py silently falls back to a pure-Python build on any compile failure otherwise.

Testing

• same as upstream

License: OK