htlcswitch+go.mod: use updated fn.ContextGuard

This commit updates the fn dep to the version containing the updates to
the ContextGuard implementation. Only the htlcswitch/link uses the guard
at the moment so this is updated to make use of the new implementation.

htlcswitch/test_utils.go

@@ -1190,7 +1190,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 			for {
 				select {
 				case <-notifyUpdateChan:
-				case <-chanLink.Quit:
+				case <-chanLink.cg.Done():
 					close(doneChan)
 					return
 				}