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.

See the contribution guide for more details.

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