Make preload survive without a checksum
Make preload survive without a checksum

Need to download to a temporary file, and only replace
the original after explicitly validating the checksum.

warmchang/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube