mujoco: add build-mujoco.yml for riscv64 wheels

[luhenry]

• Package: mujoco
• Version: 3.12.0
• Source: https://github.com/google-deepmind/mujoco
• Docs: https://mujoco.readthedocs.io/en/3.12.0

Compiles the MuJoCo physics engine plus its pybind11 Python bindings, using upstream's own two-stage CMake flow (core library, then bindings against it) rather than cibuildwheel. Upstream publishes no riscv64 wheel.

Mirrors upstream's .github/workflows/build.yml/build_steps.sh (configure_mujoco/build_mujoco/install_mujoco, then build_python_bindings against MUJOCO_PATH).

Differs from upstream

• Studio (Filament GUI, dear_imgui/implot) and OpenUSD dropped - Filament needs Vulkan/Metal tooling with no riscv64 story, USD is a separate heavy build.
• Classic GLFW viewer (mujoco.viewer) kept, mirroring the already-published glfw riscv64 port's X11/Wayland dnf packages.
• All native deps (ccd, qhull, tinyxml2, tinyobjloader, lodepng, miniz, pybind11, glfw) build from source via CMake FetchContent.

Testing

• same as upstream (pytest --pyargs mujoco), plus an mj_step smoke test.

License: OK - Apache-2.0, wheel carries upstream's LICENSE and LICENSES_THIRD_PARTY.md.

CI running; will update with pass/skip counts once the first build completes.