Automatic merge from submit-queue

Always skew the client when running version-skew tests

As mentioned in the comments:

- for upgrade jobs, we want kubectl to be at the same version as master.
- for client skew tests, we want to use the skewed kubectl (that's what we're testing).

So, remove gate on `JENKINS_USE_SKEW_KUBECTL`, and always use the skewed `kubectl`.

This should go in before https://github.com/kubernetes/test-infra/pull/109.