Store pubkeys in TRDescriptor::MakeScripts

When expanding the scripts for a TRDescriptor, also store the pubkeys in
the FlatSigningProvider.

src/script/descriptor.cpp

@@ -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