Correctly handle platforms that include CPU variants

[imjasonh]

Prior to this change, Kaniko would only select the platform-specific
image from a multi-platform image using the OS and architecture to
select the image. This leads to problems when there are two platforms
that are indistinguishable using only this information (e.g.,
linux/arm/v5 and linux/arm/v7).

This change more explicitly selects the right image from a
multi-platform image, taking CPU variant into account (v5 vs v7), using
containerd's CPU variant detection logic.

This also moves platform defaulting up as early as possible in execution
as it can go, so that malformed platform values are detected as soon as
possible.

Fixes #1920

Submitter Checklist

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• Includes unit tests
• Adds integration tests if needed.

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Reviewer Notes

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

Release Notes

Correctly handle cases where a multi-platform base image includes multiple images for the same OS+architecture, distinguished by their CPU variant (e.g., python:3.7 supports linux/arm/v5 and linux/arm/v7)