Use OpenMPI 4.0 in github checks

[holke]

Add an OpenMPI 4.0 version to the github actions to be checked automatically.

[spenke91]

@holke Did you specifically have version 4.0 in mind here for some reason? If not, I think we can close this issue now that #2055 has been merged, since the new Clang & OpenMPI github workflow uses version 4.1.6.