Must use render! for gdk-config.mk

What does this merge request do and why?

This MR fixes a bug when running gdk reconfigure that occurs when generating the gdk-config.mk file. gdk-config.mk is a special file that's included within our Makefile and when included, cannot produce any output as that will result in make deeming the action a failure.

How to set up and validate locally

⚠ Run gdk config get hostname and remember the details here.

Using the main branch

Run the following and you should see the error:

1. gdk config set hostname blah1.test ; gdk reconfigure

Using this branch (1787-changing-gdk-yml-causes-an-error-on-first-gdk-reconfigure)

Check out the 1787-changing-gdk-yml-causes-an-error-on-first-gdk-reconfigure branch and run the following and you should not see any error.

1. gdk config set hostname blah2.test ; gdk reconfigure

Now don't forget to re-set your previous hostname value:

1. gdk config set hostname <previous value> ; gdk reconfigure

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.

Closes #1787 (closed)