dev: Fix `notest` docker build branches

Created by: mrnugget

The idea behind the docker-images-patch-notest/<IMAGE> branches was that you could push to them to build a Docker image on CI without running through the test suite (hence the notest in the name).

All this change here does is move the existing code around so that the check for the branch name results in the early exit that we want.

To test I pushed this branch here to the a notest-branch:

$ git push origin docker/fix-notest-branches:docker-images-patch-notest/frontend --force

The resulting build only generates a pipeline and runs a Docker image build. See here: https://buildkite.com/sourcegraph/sourcegraph/builds/38666#efe8245a-872f-44ad-98cc-a2a71e61c853

Question: is that correct or should we run more steps?