STDOUT of powerpipe benchmark run doesn't follow --output option when a new version is available

[k-kawa]

Describe the bug
When a new version of powerpipe is available, powerpipe benchmark run --output json command prints a notification to its stdout so that the output results in an invalid JSON string.

Powerpipe version (powerpipe -v)
v0.4.0

To reproduce
Run the command with its not-latest binary.

Expected behavior
The format of the command follows --output option even if its new version is available. The notification message should be printed to its stderr not stdout.

Additional context
Add any other context about the problem here.

[pskrbasu]

Thanks @k-kawa for finding this. We will fix it asap.