Process: Update test-runner image

[wlynch]

I recently updated knative/test-infra to update githubhelp to reduce the number of API calls it makes for listing changed files - knative/test-infra#1555. I'd like to update the prow image so prow can actually run tests for tektoncd/pipeline#1607.

test-runner uses the latest clone (perhaps we should also pin this similar to #125?), so all we need to regen the test-runner image as is. I don't have permissions to do this, so someone else needs to. 🙃

[wlynch]

From slack:

It's updated nightly
the cronjob runs at 3am GMT every day, one for every image in tekton/images i.e. testrunner, tkn and skopeo