jupiterbrain: copy stdout/stderr to log writer via pipe

Apparently there's a data race when stdout and stderr is the same
writer: golang/go#5582. We're gonna attempt to work around it by letting
the library generate pipes for both stdout and stderr and then using
io.Copy to copy all of the data back to the log writer.

Author: sarahhodne

src/github.com/travis-ci/worker/backend/jupiterbrain.go

@@ -401,8 +401,18 @@ func (i *jupiterBrainInstance) RunScript(ctx context.Context, output io.Writer)
 		return &RunResult{Completed: false}, err
 	}
 
-	session.Stdout = output
-	session.Stderr = output
+	stdoutPipe, err := session.StdoutPipe()
+	if err != nil {
+		return &RunResult{Completed: false}, err
+	}
+
+	stderrPipe, err := session.StderrPipe()
+	if err != nil {
+		return &RunResult{Completed: false}, err
+	}
+
+	go io.Copy(output, stdoutPipe)
+	go io.Copy(output, stderrPipe)
 
 	errChan := make(chan error, 1)