Elementary proof that Bool is a Set

Question

The type Bool, as in the datatype with just two point constructors true false : Bool and no higher-dimensional constructors, has decidable equality which is easy to prove directly, e.g. via a type family that maps true to Unit and false to Void. Then we can apply Hedberg's theorem to show that Bool is a Set.

What is a direct, elementary proof of IsSet Bool that doesn't go via Hedberg's theorem?