Using the same (recent) Go version for all Kubernetes versions can
break for older versions when there are incompatible changes in Go. To
avoid that, we use exactly the minimum version of Go required for each
Kubernetes version. This is based on the assumption that this
combination was tested successfully.
When building the E2E suite from Kubernetes (the default) we do the
same, but still allow building it from elsewhere.
We allow the Go version to be empty when it doesn't matter.
