skip unknown platforms for hauler save manifest.json

[amartin120]

Please check below, if the PR fulfills these requirements:

• Commit(s) and code follow the repositories guidelines.
• Test(s) have been added or updated to support these change(s).
• Doc(s) have been added or updated to support these change(s).

Associated Links:

Types of Changes:

• bugfix

Proposed Changes:

• When constructing the manifest.json for use with docker load, if a multi-arch image contains a platform defined as "unknown/unknown" (looking at you, busybox), docker fails to load.

Verification/Testing of Changes:

Additional Context:

[zackbradys]

Testing current commits/state of this PR here -> https://github.com/zackbradys/rancher-airgap/actions/runs/10969610724

[amartin120]

Closing this attempt. This causes bad things.