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.

prow.sh

@@ -360,6 +360,9 @@ git_checkout () {
         (cd "$path" && run git fetch "$repo" '+refs/heads/*:refs/remotes/csiprow/heads/*' '+refs/tags/*:refs/tags/*') || die "fetching $repo failed"
         (cd "$path" && run git checkout "$revision") || die "checking out $repo $revision failed"
     fi
+    # This is useful for local testing or when switching between different revisions in the same
+    # repo.
+    (cd "$path" && run git clean -fdx) || die "failed to clean $path"
 }
 
 list_gates () (
@@ -410,10 +413,20 @@ start_cluster () {
             git_checkout https://github.com/kubernetes/kubernetes "$GOPATH/src/k8s.io/kubernetes" "$version" --depth=1 || die "checking out Kubernetes $version failed"
 
             # "kind build" and/or the Kubernetes build rules need at least one tag, which we don't have
-            # when doing a shallow fetch. Therefore we fake one. For some reason, v1.0.0 didn't work
-            # while v1.14.0-fake.1 did.
-            (cd "$GOPATH/src/k8s.io/kubernetes" && run git tag -f v1.14.0-fake.1) || die "git tag failed"
-
+            # when doing a shallow fetch. Therefore we fake one:
+            # release-1.12 -> v1.12.0-release.<rev>.csiprow
+            # latest or <revision> -> v1.14.0-<rev>.csiprow
+            case "${CSI_PROW_KUBERNETES_VERSION}" in
+                release-*)
+                    # Ignore: See if you can use ${variable//search/replace} instead.
+                    # shellcheck disable=SC2001
+                    tag="$(echo "${CSI_PROW_KUBERNETES_VERSION}" | sed -e 's/release-\(.*\)/v\1.0-release./')";;
+                *)
+                    # We have to make something up. v1.0.0 did not work for some reasons.
+                    tag="v1.14.0-";;
+            esac
+            tag="$tag$(cd "$GOPATH/src/k8s.io/kubernetes" && git rev-list --abbrev-commit HEAD).csiprow"
+            (cd "$GOPATH/src/k8s.io/kubernetes" && run git tag -f "$tag") || die "git tag failed"
             go_version="$(go_version_for_kubernetes "$GOPATH/src/k8s.io/kubernetes" "$version")" || die "cannot proceed without knowing Go version for Kubernetes"
             run_with_go "$go_version" kind build node-image --type bazel --image csiprow/node:latest --kube-root "$GOPATH/src/k8s.io/kubernetes" || die "'kind build node-image' failed"
             csi_prow_kind_have_kubernetes=true