Set gitlab shell origin under tmp/tests

so that developers can temporarily change the origin to their fork
and still have the CI pass even if tmp/tests/gitlab-shell is cached
between builds.

Before this PR, there is no way to let the CI run your tests if you modify gitlab-shell, because:

- the CI caches tmp/gitlab-shell between builds
- you want to make it point to your own modified fork of gitlab shell to ensure that tests will pass with the new shell

But that was not possible since the origin was fixed to the existing cached one.

Now it is explicitly set.