Add KANIKO_REGISTRY_MIRROR env var

[imjasonh]

Fixes #1752

Description

This adds support for checking a KANIKO_REGISTRY_MIRROR env var. If the env var is set, any value passed via --registry-mirror is ignored, and the env var value is used instead.

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• Includes unit tests
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• The code flow looks good.
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Release Notes

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Add KANIKO_REGISTRY_MIRROR env var to override/set --registry-mirror flag