write stderr from the subprocess in lb-wrapcmd to stdout
This way the problem with G4 builds hanging after the compilation is gone.
I wonder if this has something to do with the fact that since Python 3.9, sys.stderr is line-buffered.
We do not lose the information about stderr while presenting the logs in the frontend, because it's sent separately from stdout to the socket. It's just that they are indistinguishable in the log file.