Automatic merge from submit-queue

Don't git clean in hack/jenkins/build.sh

We go through [all the effort](https://github.com/kubernetes/test-infra/pull/521) of activating a service account but then immediately wipe it away in the build script.

We do a full workspace cleanup anyway, so we don't need the `git clean`. We could probably remove the other cleanup lines in this script, too, but I wanted to keep this change as small as possible for now.

Maybe actually fixes https://github.com/kubernetes/test-infra/issues/470?