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

[mzihlmann]

migrated from #3338 here to unblock main of my fork.

#3088 (comment)

doesn't fix #3088 but is related to that issue

Description

Currently the CreatedAt timestamp (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, 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 CreatedAt field in base images