GitHub Actions: Avoid Ubuntu 20.04 for coverage collection
GitHub Actions: Avoid Ubuntu 20.04 for coverage collection

.. because its MinGW GCC 9.3.0 fails to create any .gcda files
at test runtime (while that works just fine on 18.04 and 22.04).

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat