Add shellcheck and others to prow-tests image (#1979)
Add shellcheck and others to prow-tests image (#1979)

* Add shellcheck and others to prow-tests image

The former for adding a presubmit
The others for being prepared for future use from the image (currently
they are used directly via "go run" in the scripts).

* Don't install kntest
knative/test-infra,knative/test-infra,knative/test-infra