pydantic-monty-runtime: add build-pydantic-monty-runtime.yml for riscv64 wheels

[luhenry]

• Package: pydantic-monty-runtime
• Version: 0.0.21
• Source: https://github.com/pydantic/monty
• Docs: https://github.com/pydantic/monty

Compiles a maturin bindings = "bin" wheel wrapping the monty CLI binary, the sandboxed Python interpreter's REPL/file-runner/subprocess-worker entry point. Upstream publishes no riscv64 wheel.

Mirrors the build job of upstream's own ci.yml.

Differs from upstream

• Builds once with no -i interpreter list, unlike upstream's -i 3.10 3.11 3.12 3.13 3.14 - the binary has no ABI dependency, so this yields one interpreter-agnostic wheel instead of five near-duplicates.
• Copies the workspace-root LICENSE beside the crate's pyproject.toml before building; upstream's own published wheel ships without it.
• Targets riscv64gc-unknown-linux-gnu with manylinux: '2_39', not one of upstream's own architectures (which build on manylinux_2_28).

Testing

• No wheel-level test suite ships in the sdist; drives the real binary against fixtures mirroring upstream's own tests/mounts.rs (script execution, a read-write mount round trip, an unmounted-path PermissionError).

License: OK - MIT, nothing third-party vendored.