Add the ability to skip unpacking the initial file system

[natalieparellano]

Description

Add the ability to skip unpacking the initial file system

• Adds a new option, InitialFSUnpacked
• When opts.InitialFSUnpacked is true, the first stage builder will
  skip unpacking the file system; later stages are unaffected

Why do this:
This only makes sense if kaniko is used as a library, and the executor is running
in the context of an image that is the image referenced in the first FROM statement
in the Dockerfile. buildpacks.io is aiming to do this in order to customize base images
prior to buildpacks builds.

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

Describe any changes here so maintainer can include it in the release notes, or delete this block.

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