Fix bug handling images with slashes in ref

[puerco]

Summary

This commit fixes a bug when trying to list images with names in their reference. The github API expects the path segments after the org to be url escaped, for example:

 `ghcr.io/org/name/with/slahes`

needs to be

 `ghcr.io/org/name%2Fwith%2Fslahes`

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

TBD

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 50.517% (+0.003%) from 50.514%
when pulling c06ebd5 on puerco:urlencode-bug
into 7c50e65 on stacklok:main.