prow.sh: avoid AllAlpha=true
prow.sh: avoid AllAlpha=true

It turned out to not work. Instead of reverting the commit which
introduced this, let's better document this explicitly.

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