Resolve guix non-determinism with emplace_back instead of push_back

For some reason, building x86_64-w64-mingw32 on x86_64 and aarch64
results in a single instruction difference which can be traced down to
prevector.h:174. The ultimate caller of this is the copy constructor for
a prevector that ends up being called by std::vector::push_back in
walletmodel.cpp:183. By replacing the push_back with an emplace_back,
somehow this non-determinism goes away.

src/qt/walletmodel.cpp

@@ -179,8 +179,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             setAddress.insert(rcp.address);
             ++nAddresses;
 
-            CRecipient recipient{DecodeDestination(rcp.address.toStdString()), rcp.amount, rcp.fSubtractFeeFromAmount};
-            vecSend.push_back(recipient);
+            vecSend.emplace_back(CRecipient{DecodeDestination(rcp.address.toStdString()), rcp.amount, rcp.fSubtractFeeFromAmount});
 
             total += rcp.amount;
         }