Don't skip pull by default

When I added QA_SKIP_PULL it seems I didn't consider the consequences of making it true by default. Or maybe I didn't realize 😱

That means if someone used gitlab-qa it wouldn't automatically fetch the latest image, which could lead to unexpected inconsistencies in tests. E.g., tests failing because they use an old image that we expected to be updated.

It typically wasn't a problem in CI because the image didn't exist and so docker pulled anyway (when it tried to run the image).

Note: This might affect local workflows because after this gitlab-qa will pull unless QA_SKIP_PULL is true.