The node finder should use `button_released(MouseButton::Secondary)` when available

[setzer22]

In #40, a change was introduced where the check to spawn the node finder used mouse.button_released instead of mouse.secondary_down(). This is the more correct behavior, as you only want to spawn the finder once the click is complete. Unfortunately, this is only available on egui master, not 0.18, so this change cannot be applied until egui 0.19 is released.

I'm leaving this issue as a reminder