There are use cases, where multiple Go build flags need to be set. However, the
environment variable to pass flags to Go build has some limits for `ldFlags`.
Add GoReleaser inspired configuration section to `.ko.yaml` to support setting
specific Go build and ldFlags to be used by the build. Like GoReleaser the
content of the configuration can use Go templates. Currently, only a section
for environment variables is included.
In order to reduce dependency overhead, only the respective config structs from
https://github.com/goreleaser/goreleaser/blob/master/pkg/config/config.go are
used internally to load from `.ko.yaml`.
