core: use an advice to write custom settings to dotfile

Should fix #7816

core/core-custom-settings.el

@@ -92,4 +92,6 @@ This function is called at the very end of Spacemacs initialization.\"\n")
       (save-buffer)
       (kill-buffer (current-buffer)))))
 
+(advice-add 'custom-save-all :after #'spacemacs/write-custom-settings-to-dotfile)
+
 (provide 'core-custom-settings)

core/core-spacemacs.el

@@ -78,7 +78,6 @@ the final step of executing code in `emacs-startup-hook'.")
                 ;; overlapped in terminal mode. The GUI specific `<C-i>' is used
                 ;; instead.
                 evil-want-C-i-jump nil)
-  (spacemacs/write-custom-settings-to-dotfile)
   (dotspacemacs/load-file)
   (require 'core-configuration-layer)
   (dotspacemacs|call-func dotspacemacs/init "Calling dotfile init...")