(Backport) Issue #7613: Prevent thumbnail from being added on export override

This patch attempts to resolve #7613 (closed) on 2.10 (as it is already fixed on 2.99).

It implements @Wormnest's fix with his suggestions (gimp_export_thumbnail () is treated as TRUE and thus not included). Code was tested by exporting and overwriting with both the "Save Thumbnail" checkbox toggled on and off, and seemed to behave as expected.