refactor: Make const refs vars where applicable

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

src/external_signer.cpp

@@ -28,7 +28,7 @@ bool ExternalSigner::Enumerate(const std::string& command, std::vector<ExternalS
     if (!result.isArray()) {
         throw std::runtime_error(strprintf("'%s' received invalid response, expected array of signers", command));
     }
-    for (UniValue signer : result.getValues()) {
+    for (const UniValue& signer : result.getValues()) {
         // Check for error
         const UniValue& error = find_value(signer, "error");
         if (!error.isNull()) {