Skip test collation for LLVM builds on Windows

No sure why, but lcov fails in this scenario, at least on GitHub
hosted runners:

> error: Failed to load coverage: 'D:/a/_temp/coverage/src/lib/
> CMakeFiles/QtPokit.dir/QtPokit_autogen/mocs_compilation.cpp.obj':
> Malformed coverage data
> error: Could not load coverage information

.github/workflows/build.yaml

@@ -615,7 +615,7 @@ jobs:
         if: matrix.arch != 'arm64'
         run: ctest --output-on-failure --test-dir "%RUNNER_TEMP%/release" --verbose
       - name: Collate test coverage
-        if: matrix.toolchain != 'msvc'
+        if: matrix.toolchain != 'msvc' && matrix.toolchain != 'llvm'
         run: cmake --build "%RUNNER_TEMP%/coverage" --target coverage --verbose
       - name: Upload test results
         if: matrix.arch != 'arm64' && ( matrix.toolchain != 'msvc' || fromJSON(steps.post-build.outputs.tap) )