fix(KanikoDir): update DOCKER_CONFIG env when use custom kanikoDir

[chenggui53]

Description

In v1.9.0, when use custom KanikoDir, kaniko will copy all from kanikoDefauiltDir(/kaniko) to custom KanikoDir，and then remove all in KanikoDefaultDir(/kaniko), and kaniko defines DOCKER_CONFIG as /kaniko/.docker in Dockerfile. So user can't get auth if uses custom kanikoDir.
To avoid this, I added a section that updates environment variables if use custom Kaniko dir, so kaniko can get correct auth from new dir.

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Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

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