clang-cl actions don't use clang-cl with CMake

[cmazakas]

The CMAKE_*_COMPILER values are correct but to coerce CMake into using clang-cl, one must append -T ClangCL in the invocation.

I hacked this in:

extra-args: -D Boost_VERBOSE=ON -D BUILD_TESTING=ON -D BOOST_SRC_DIR="../boost-root" ${{ matrix.compiler == 'clang-cl' && '-T ClangCL' || '' }}