gitlab-ci: Fix pipeline on non-work-in-progress branches

The ${source_wip:=false} snippet not only sets the $source_wip variable if unset, but also executes the result of its expansion.

In our case it executed false, which made the pipeline very unhappy.

However, all tests submitted from this repository directly should be considered "wip" and it will be up to the image repository including the gitlab-ci job template to define the logic for which submissions should be "wip" or not.

So drop the logic and use a plain source_wip variable defaulting to true. The meaning of the variable was not documented either, so add a blurb about it.