minijinja: add build-minijinja.yml for riscv64 wheels

[luhenry]

• Package: minijinja
• Version: 2.24.0
• Source: https://github.com/mitsuhiko/minijinja
• Docs: https://github.com/mitsuhiko/minijinja

Compiles minijinja-py, the PyO3 bindings crate for MiniJinja, a Rust implementation of Jinja2 templating. Upstream publishes no riscv64 wheel.

Mirrors the linux job of upstream's build-wheels.yml.

Differs from upstream

• musllinux dropped - rustup ships no riscv64 musl toolchain.
• Rust toolchain installed in-container rather than on the runner.

Matrix: cp38-abi3 (pyo3's abi3-py38 feature is unconditional; built on cp39, the riscv64 image's floor, the same interpreter upstream's own workflow builds with despite the same cp38-abi3 tag) plus cp314t.

Testing

• same as upstream (pytest across minijinja-py/tests)

License: OK

Built on cp39-abi3 and cp314t; 46 passed on both (verified locally on both legs before pushing).