Add cmake, libclang-dev and a shared-library Python to the runner image #96

A Rust job that builds C/C++ sys crates (a coin-or solver crate through the cc crate, HiGHS through cmake and bindgen) currently installs cmake and libclang-dev with apt on every run, and downloads a CPython build because pyo3 links libpython into the test binary, and the image's Python 3.12 ships no shared library (the link fails with unable to find library -lpython3.12).

Observed per run: the apt install pulls libllvm, libclang and cmake-data, and the Python download is a separate fetch every time. Both are small next to the compile, but they are per-job fixed cost on every run of that job.

Adding cmake, libclang-dev and a CPython with its shared library (or a python3-dev equivalent) to the image would let those install steps be no-ops. The job's steps are guarded (command -v and ldconfig -p checks), so they stay portable to GitHub-hosted runners either way.