Use canonical platform values. Fix 1995.

[gadiego92]

Fixes #1995

Description

Prior to this change, Kaniko was not using the platform's canonical value.
On this PR Normalize function from containerd module is used to get the canonical platform. To get the platform value DefaultSpec has been replaced by DefaultSpec.

Submitter Checklist

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• Includes unit tests
• Adds integration tests if needed.

Reviewer Notes

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

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