Add canonical image fallback to integration job

What does this merge request do and why?

Currently, the build-integration-image CI job relies on there being a verify container image available to use in the pipeline project as base image.

On forks and especially the community fork, the job usually fails because they don't build the verify image (regularly). This requires triggering a pipeline in the canonical GDK project when reviewing an MR.

This behavior is annoying both for community contributions as well as dependency upgrades. Therefore, this allows the build-integration-image job to fall back to the canonical verify image, which should be continuously available.

How to set up and validate locally

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.