Fix CodeQL workflow: drop update-alternatives, pass gcc-14 directly to cmake

[Copilot]

lib/xml/xmlparser.cpp uses #include <generator> (C++23 std::generator), available only in gcc-14+. The previous fix installed gcc-14 but routed it through update-alternatives to override system symlinks — a fragile indirection that also had no effect on GitHub's parallel dynamic code scanning run, which uses its own autobuild runner.

Changes

• Removed the "Select gcc-14/g++-14 as default compiler" step (update-alternatives across all four gcc/g++/cc/c++ aliases)
• Removed CC/CXX env vars from the Build step
• Pass compiler paths directly to cmake — no system-wide symlink mutation needed:

- name: Build
  run: |
    cmake -B _build -DCMAKE_C_COMPILER=/usr/bin/gcc-14 -DCMAKE_CXX_COMPILER=/usr/bin/g++-14
    cmake --build _build --parallel $(nproc)

Note on the dynamic scan

The linked failing job (runs/31785672753) is GitHub's default code scanning (dynamic/github-code-scanning/codeql), which runs independently of this workflow using autobuild with system gcc-13. That cannot be fixed via repository files. To stop it: Settings → Security → Code scanning → Default setup → Disable. Our custom workflow already handles analysis correctly.

[Copilot]

Pull request overview

Updates the repository’s custom CodeQL workflow to reliably use GCC 14 for the C/C++ build without mutating system-wide compiler symlinks, aligning the workflow with the project’s C++23 requirement (<generator>).

Changes:

• Removed the update-alternatives step that globally rewired gcc/g++/cc/c++.
• Removed CC/CXX environment overrides in the build step.
• Configured CMake to use /usr/bin/gcc-14 and /usr/bin/g++-14 directly via -DCMAKE_*_COMPILER.

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