Fix stderr redirection (jesseduffield#3479)

- **PR Description**

Seems that there's a problem in the Stdout/Stderr/Stdin stream vars
assignments,
probably copy-paste issue.

If this is intentional, I suggest adding an explanation on why `Stderr`
-> `Stdout` intentionally to avoid other PRs and confused people around
:)

Thanks!

pkg/gui/gui.go

@@ -813,7 +813,7 @@ func (gui *Gui) runSubprocess(cmdObj oscommands.ICmdObj) error { //nolint:unpara
 
  subprocess := cmdObj.GetCmd()
  subprocess.Stdout = os.Stdout
- subprocess.Stderr = os.Stdout
+ subprocess.Stderr = os.Stderr
  subprocess.Stdin = os.Stdin
 
  fmt.Fprintf(os.Stdout, "\n%s\n\n", style.FgBlue.Sprint("+ "+strings.Join(subprocess.Args, " ")))