Allow Akonadi tests to run on Linux.

Hopefully these work okay and don't end up causing too many issues, but at the very least it should be contained to 2 Docker containers and won't block other projects building too badly.

CCMAIL: faure@kde.org
2 jobs for master
Status Job ID Name Coverage
  Build
manual #4516
docker allowed to fail manual
kdeorg/appimage-base
manual #4517
docker allowed to fail manual
kdeorg/staticweb