ko: ko stops building containers after Golang updates

After each minor Golang version bump (e.g. 1.13.2 -> 1.13.3), ko apply no longer builds container images but happily applies the yaml untransformed, which leads with image pull errors on the cluster. After reinstalling ko, it works again as expected.

Ideally ko would continue to work after a Golang update, but if not, it should fail loudly and demand to be reinstalled.