Automatic merge from submit-queue

Add staging/ to gopath in copy.sh

An alternative to https://github.com/kubernetes/kubernetes/pull/40731.

Please check the last 2 commits of this branch out for an example run: https://github.com/caesarxuchao/kubernetes/tree/add-stagingto-gopath. I added package "validationchao" to apimachinery, and let k8s.io/kubernetes/pkg/client/generated_clientset/ depend on it, then I run hack/update-staging-client-go.sh, and it seems to work fine.

@sttts @deads2k @lavalamp