Store the output hashes in the initialOutputs of the drv goal

That way we
1. Don't have to recompute them several times
2. Can compute them in a place where we know the type of the parent
  derivation, meaning that we don't need the casting dance we had before

src/libstore/build/derivation-goal.hh

@@ -37,6 +37,7 @@ struct InitialOutputStatus {
 
 struct InitialOutput {
     bool wanted;
+    Hash outputHash;
     std::optional<InitialOutputStatus> known;
 };