ci: do not wait for images to build before starting tests

Created by: bobheadxi

From what I understand, the goal of https://github.com/sourcegraph/sourcegraph/pull/13689 is to start to build the images earlier, but it seems that what it does is cause tests to wait until images build. This PR is more or a proposal because I feel like we still want tests to try to grab an agent to start running on (while still prioritizing the images) - I think this change does the trick?

There is already a wait before the publish step, so this change should still wait for all images to build before publishing

Currently:

It seems that all the docker images must build before tests start