Skip unpacking optimization breaks COPY in multi-stage builds

[aduong]

Actual behavior
Because of #440, it's possible for a later step to COPY files from an unpacked build and fail because the file does not exist in the /kaniko working area.

Expected behavior
COPYing from any previous build stage should be fine if the file should exist in the stage.

To Reproduce
Run Kaniko on

FROM python:stretch
FROM python:stretch
COPY --from=0 /bin/bash /bin/bash

Admittedly, this particular use case can be handled by COPY --from=python:stretch (#417), but #440 also affects cached RUNs.

[aduong]

I'm mistaken. I was experiencing a different problem related to hardlinks.