Enable spinner for gdk update

What does this merge request do and why?

This enables the spinner for gdk update, so we can finally parallelize it in !3726 (merged).

This acts an MR that we can easily revert if there are issues.

Related to #2115 (closed)

How to set up and validate locally

1. Run GDK_SELF_UPDATE=0 gdk update
2. See the new spinner in action 🎉

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 change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.