productbuild: Capture stderr too in CPackProductBuild logs

Stdout and stderr will be merged.

Fixes: #18234

Source/CPack/cmCPackProductBuildGenerator.cxx

@@ -144,10 +144,10 @@ bool cmCPackProductBuildGenerator::RunProductBuild(const std::string& command)
   tmpFile += "/ProductBuildOutput.log";
 
   cmCPackLogger(cmCPackLog::LOG_VERBOSE, "Execute: " << command << std::endl);
-  std::string output, error_output;
+  std::string output;
   int retVal = 1;
   bool res = cmSystemTools::RunSingleCommand(
-    command.c_str(), &output, &error_output, &retVal, nullptr,
+    command.c_str(), &output, &output, &retVal, nullptr,
     this->GeneratorVerbose, cmDuration::zero());
   cmCPackLogger(cmCPackLog::LOG_VERBOSE, "Done running command" << std::endl);
   if (!res || retVal) {