Absolutely NO risk.
If this was a risk, the entire world would be infected.
You can NOT get HIV from exposure to fluids (even infected) from environmental surfaces. Why? Because the virus is so fragile, it can not maintain its infectiousness outside of the body.
Plus, a thumbtack stick would not be a sufficient portal of entry to allow enough blood in to reach the bloodstream to cause infection, which has to happen.
Lastly, even IF the virus would maintain its effectiveness on the thumbtack, there wouldn't be enough blood to constitute a risk.
Don't give this another thought.