message: Store pubkeys in TRDescriptor::MakeScripts
  When expanding the scripts for a TRDescriptor, also store the pubkeys in
  the FlatSigningProvider.
diff: @@ -851,6 +851,7 @@ protected:
  builder.Finalize(xpk);
  WitnessV1Taproot output = builder.GetOutput();
  out.tr_spenddata[output].Merge(builder.GetSpendData());
  + out.pubkeys.emplace(keys[0].GetID(), keys[0]);
  return Vector(GetScriptForDestination(output));
  }
  bool ToStringSubScriptHelper(const SigningProvider* arg, std::string& ret, const StringType type, const DescriptorCache* cache = nullptr) const override
  