Update filename for GDK CI pipelines config in gdk_in_a_box.md

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What does this merge request do and why?

This MR updates the filename for the GDK CI pipelines config. .gitlab/ci/build.gitlab-ci.yml does not exit but rather .gitlab/ci/build-images.gitlab-ci.yml

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.