Deprecate usage of KO_DOCKER_REPO

[imjasonh]

Relying on this env var makes sense for ko as a standalone tool. In IaC it makes less sense.

A more idiomatic solution would be to use the provider-wide attribute fed by a variable, which can be fed by a variables file, or an environment variable if desired.

We should warn when KO_DOCKER_REPO is found to be load-bearing, and eventually remove the option for it to be used this way.

We might still want to warn if KO_DOCKER_REPO is set and we use the provider-specified value which is different, since this could be confusing for users using ko directly alongside the TF provider.