Ensure kubernetes-src.tgz has kubernetes/ prefix

There's a difference between the kubernetes-src.tgz build from
`make bazel-release` and `make quick-release`. The quick-release does
not have a `kubernetes/` prefix and hence essentiall tarbomb(s) the
directory when someone tries to untar it.

Change-Id: I8e87639d85dd01aec534b58f1d5740bd48ac922f

build/lib/release.sh

@@ -115,7 +115,7 @@ function kube::release::package_src_tarball() {
           \) -prune \
         \))
     )
-    "${TAR}" czf "${src_tarball}" -C "${KUBE_ROOT}" "${source_files[@]}"
+    "${TAR}" czf "${src_tarball}" --transform 's|^\.|kubernetes|' -C "${KUBE_ROOT}" "${source_files[@]}"
   fi
 }