engine: graph: Avoid race on fast pause variable

This code is basically unused, but let's keep it in for now in case we
eventually replace it with some contextual ctx code instead.

engine/graph/actions.go

@@ -376,7 +376,7 @@ func (obj *Engine) Process(ctx context.Context, vertex pgraph.Vertex) error {
 			// so that the graph doesn't go on running forever until
 			// it's completely done. This is an optional feature and
 			// we can select it via ^C on user exit or via the GAPI.
-			if obj.fastPause {
+			if obj.fastPause.Load() {
 				obj.Logf("%s: fast pausing, poke skipped", res)
 				continue
 			}