regress: add build-regress.yml for riscv64 wheels

[luhenry]

• Package: regress
• Version: 2025.10.1
• Source: https://github.com/crate-py/regress
• Docs: https://github.com/crate-py/regress

Compiles a PyO3 extension wrapping the Rust regress ECMAScript regular expression crate. Upstream publishes no riscv64 wheel.

Mirrors upstream's CI.yml manylinux job.

Differs from upstream

• Builds with cibuildwheel rather than maturin-action, which has no riscv64 target.
• Installs the Rust toolchain in the build container.
• musllinux dropped - rustup ships no riscv64 musl toolchain.

Testing

• same as upstream

License: OK

Built on cp312; 6 passed.