kivy: confirm_tx_dialog: toggling "final" should update the tx

We create a tx when the dialog is created, and re-create it every time
the user moves the fee slider. However, we were not re-creating it if
the user toggles the "Final" checkbox, meaning its value was only taken
into account if the user moved the fee slider after setting the checkbox.

fixes https://github.com/spesmilo/electrum/issues/7547

electrum/gui/kivy/uix/dialogs/confirm_tx_dialog.py

@@ -93,6 +93,7 @@ Builder.load_string('''
                 id: final_cb
                 opacity: int(root.show_final)
                 disabled: not root.show_final
+                on_release: root.update_tx()
         Label:
             text: root.warning
             text_size: self.width, None