Make go and kubectl version configurable

[embano1]

Currently, ko uses hardcoded values to retrieve the go and kubectl executables, e.g.

ko/pkg/build/gobuild.go

Line 288 in 453bf80

cmd := exec.CommandContext(ctx, "go", args...)

The desired behavior is to make those binary locations configurable, e.g. via KO_GO_PATH and KO_KUBECTL_PATH.
Alternatively, they could be specified in .ko.yaml but this might be overkill as setting these values are considered exceptional.

[imjasonh]

Let's start with KO_GO_PATH. AFAIK nobody's asked to be able to change the kubectl path. The naming is convenient because if we want to make it configurable in .ko.yaml we can just add a go_path field and Viper will pick up KO_GO_PATH for us.