Stabilize CodeQL by pinning default compiler aliases to gcc-14

[Copilot]

The Analyze (c-cpp) job was still resolving the build through the runner’s generic compiler entrypoints, which allowed image drift to bypass the intended gcc-14/g++-14 toolchain. That made the CodeQL build unstable and led to recurring failures on #include <generator> despite the project compiling correctly on known-good environments.

• Root cause

  • The workflow installed gcc-14 / g++-14, but did not force the generic aliases cc and c++.
  • In production, the CodeQL build path was still going through c++, so the effective compiler could change with the runner image.

• What changed

  • Force all compiler entrypoints used by the job to the same toolchain:
    • gcc -> gcc-14
    • g++ -> g++-14
    • cc -> gcc-14
    • c++ -> g++-14
  • Update the CMake invocation to build through cc / c++ instead of naming gcc-14 / g++-14 directly, so the workflow follows the exact compiler path CodeQL uses in practice.

• Why this is more stable

  • The workflow now controls both the explicit compiler binaries and the generic aliases the runner may use internally.
  • This removes dependence on whatever cc / c++ happen to point to after a GitHub runner image update.

• Workflow shape

  sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc-14 140
  sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++-14 140
  sudo update-alternatives --set cc /usr/bin/gcc-14
  sudo update-alternatives --set c++ /usr/bin/g++-14
  
  cmake -B _build -DCMAKE_C_COMPILER=cc -DCMAKE_CXX_COMPILER=c++

[clauspruefer]

If that does not work, we will get a problem.

[Copilot]

Pull request overview

This PR stabilizes the CodeQL Analyze (c-cpp) workflow by ensuring the generic compiler entrypoints (cc/c++) are pinned to the intended gcc-14/g++-14 toolchain, preventing GitHub runner image drift from changing the effective compiler used during CodeQL builds.

Changes:

• Add update-alternatives entries and explicit --set calls for /usr/bin/cc and /usr/bin/c++ to point to gcc-14 / g++-14.
• Update the build step to use CC=cc, CXX=c++, and configure CMake with -DCMAKE_C_COMPILER=cc -DCMAKE_CXX_COMPILER=c++.
• Expand diagnostic output to show which and version output for gcc, g++, cc, and c++.

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