CI builds of qt with unity builds

Unity builds of qt often break, because they are not validated on CI. it would be great to have a set of builds (pre-integration with gerrit or on a slower cadence e.g. nightly/weekly builds) that validate unity builds (ideally both building and running unit tests)

in my experience it would be good to have macos/win/linux builds as they often break on one platform but not on others, because e.g. a specific platform header pollutes the preprocessor.