[codex] add sccache to CI

[lovasoa]

Summary

Enable sccache in GitHub Actions so CI can cache Rust compilation outputs and CMake-driven C/C++ compilation outputs across runs.

Changes

• install mozilla-actions/sccache-action in both workflow jobs
• set RUSTC_WRAPPER=sccache and SCCACHE_GHA_ENABLED=true
• set CMAKE_C_COMPILER_LAUNCHER=sccache and CMAKE_CXX_COMPILER_LAUNCHER=sccache through the workflow environment
• print sccache --show-stats at the end of each job for visibility

Why

Most of the expensive CI time is spent compiling the bundled HiGHS C++ sources. Swatinem/rust-cache helps with Cargo downloads and dependency artifacts, but sccache is the relevant cache layer for repeated compiler invocations from both Rust and the CMake build.

Validation

• local: cargo test
• CI: not yet run on this branch at PR creation time