Allow ko layers to stack cleanly

Sometimes I'd like an image with two go binaries built with ko.

ko currently adds uniquely named binaries to to /ko-app folder but it dumps all kodata into the same KO_DATA_PATH. Thus one binary's kodata could override the other.

If each binary had it's data in a unique place setting WorkDir could be one work around - https://github.com/google/ko/issues/55

Though I still think you would need a launcher/shim binary to set the correct WorkDir prior to running the executable we want.