fix: sourceImage's labels should not be included in cache key

[mzihlmann]

Fixes #3088 relates to !3338

Description

Currently the CreatedAt timestamp, image labels (and all other metadata) is included into kaniko's cache key, this means that if a new image is created with the exact same layers but different labels, it invalidates caches in all builds that use it as a base.

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.

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Reviewer Notes

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

Release Notes

• kaniko cache learned to ignore labels in base images