refactor: Make const refs vars where applicable

This avoids initializing variables with the copy-constructor of a
non-trivially copyable type.

src/wallet/rpc/spend.cpp

@@ -1407,7 +1407,7 @@ RPCHelpMan sendall()
             }
 
             CAmount output_amounts_claimed{0};
-            for (CTxOut out : rawTx.vout) {
+            for (const CTxOut& out : rawTx.vout) {
                 output_amounts_claimed += out.nValue;
             }