Update _client with _client() to avoid to fail when creating a CSV

[spinillos]

It changes _client with _client() to make CSV works with last puppeteer update.

Fixes: https://github.com/grafana/grafana-image-renderer/issues

[Clarity-89]

Thanks for fixing this! 🎉

[Bujupah]

_client is a private function and I think it is not recommended doing it that way.
I'd recommend switching to this

    const client = await page.target().createCDPSession();
    await client.send('Page.setDownloadBehavior', {
      behavior: 'allow',
      downloadPath,
    });

@spinillos @Clarity-89