prow.sh: fix installing Go for Kubernetes 1.19.0

Kubernetes 1.19.0 uses Go 1.15, but refers to it as 1.15.0. This broke
both the check whether we need to install 1.15 (because "go version"
reports 1.15, which didn't match 1.15.0) and then downloading the
release archive (because the URL also only uses 1.15).

prow.sh

@@ -513,6 +513,8 @@ go_version_for_kubernetes () (
     if ! [ "$go_version" ]; then
         die "Unable to determine Go version for Kubernetes $version from hack/lib/golang.sh."
     fi
+    # Strip the trailing .0. Kubernetes includes it, Go itself doesn't.
+    go_version="$(echo $go_version | sed -e 's/\.0$//')"
     echo "$go_version"
 )