g++-9 with CXX2a failes (sometimes)

Apparently the docker image for g++-9 with C++20 support failes for some tests.

See for example:

• https://gitlab.dune-project.org/kilian.weishaupt/dune-grid/-/jobs/175885
• https://gitlab.dune-project.org/core/dune-istl/-/jobs/175790

I tried to reproduce the problem locally, but it works for me...