When set, use CHROME_BIN as Puppeteer ExecutablePath
grunt-contrib-qunit 3.0.1 comes with Puppeteer 1.7.0 which downloads
chrome from a third party. That can be bypassed by setting the
environment variable PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=1, user then has
to configure pupetteer ExcecutablePath.
CI containers set CHROME_BIN=/usr/bin/chromium, use that value when it
Once grunt-contrib-qunit switches to Puppeteer 1.8.0+, we can revert
this change and have CI to set: