Fix composite cache key for multi-stage copy command

[gilbsgilbs]

Fixes #1706

Description

Fix composite cache key for multi-stage copy command (#1706)

PR #1518 reintroduced COPY layers caching using the `--cache-copy-layers`
flag. Unfortunately, this PR also introduced a bug by not including the
stage digest into the caching key of the COPY command when the
`--cache-copy-layers` flag was not set. As a result, kaniko would use
any previous (possibly stalled) layer from the cache because the digest
of the "COPY --from" command would never change.

PR author probably expected Go to fallthrough in the switch just like C
does. However, this is not the case. Go does not fallthrough in
switch-statements by default and requires the fallthrough keyword to be
used. Note that this keyword is not available in type-switches though,
because it wouldn't work properly with typings.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

Author note: I did not include an integration test because you'd have to build two different images and check that the two digests differ, which is a quite different paradigm from the existing caching integration tests that always check that the digests match. I thought it would be overkill to design a new integration test system to support such scenario given the use-case is rather specific and the bug itself is kind of dummy. I don't think it'd be worth it.

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- Fix composite cache key for multi-stage copy command (#1706)

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[Cave-Johnson]

Has this regressed in the latest version of kaniko? Since the release of 1.8 I have started to experience the cache layers not detecting repository code changes