Stabilize windows-latest build by removing conflicting Qt environment state

[Copilot]

The windows-latest-build job was failing at link time due to Qt symbol mismatches, caused by Windows runner Qt environment variables overriding the intended vcpkg/MinGW dependency resolution. This change ensures CMake config on Windows starts from a clean Qt environment.

• Workflow hardening (Windows configure path)

  • Added a Windows-only step in .github/workflows/cmake.yml before Configure CMake.
  • Clears Qt5_DIR, Qt6_DIR, QTDIR, and CMAKE_PREFIX_PATH if present, preventing host-runner Qt contamination.

• Impact

  • Keeps dependency resolution aligned with the selected vcpkg triplet/toolchain in CI.
  • Avoids mixed Qt headers/libs that produced unresolved Qt symbols in windows-latest-build.

- name: Clear conflicting Qt environment (Windows)
  if: runner.os == 'Windows'
  shell: pwsh
  run: |
    @('Qt5_DIR', 'Qt6_DIR', 'QTDIR', 'CMAKE_PREFIX_PATH') | ForEach-Object {
        if (Test-Path "env:$_") {
            Remove-Item "env:$_"
        }
    }