Handle cases where multi-platform images are retagged in a mp step

[bluesliverx]

Description

Handle retagged image validation when the retagging step is also multiplatform.

Related Issue

Motivation and Context

The multiplatform retagging validation did not account for multiplatform steps doing the retagging, which is a valid use case.

How Has This Been Tested?

Unit tests.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.