And the answer is no, not anymore so than your average rock. Because the type of radioactive decay that you'd be looking for is from isotopes with very long lifetimes. Thus there would be an insignificant drop in radioactivity levels during the time it took for the lave to cool. This also means that the lava flow would remain radioactive for long periods after the eruption which would have a noticeable effect on vegetation near the boundaries of the flow. Nothing of the type has ever been noted around lava flows.