prow.sh: improve building Kubernetes from source
prow.sh: improve building Kubernetes from source

While switching back and forth between release-1.13 and release-1.14
locally, there was the problem that the local kind image kept using
the wrong kubelet binary despite rebuilding from source. The problem
went away after cleaning the Bazel cache. Exact root cause unknown,
but perhaps using unique tags and properly cleaning the repo helps.

If not, then the unique tags at least will show what the problem is.

kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-lib-utils,kubernetes-csi/external-attacher-csi,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-driver-nfs