Use ChromeHeadless for the 11.2 channel manager build

With the new pipeline/docker builds the 11.2 build is failing as it tries to start Chrome using a display. 10.2 Karma config is using PhantomJS, 12.x is using ChromeHeadless

browsers: customKarma.browsers || [config.debug ? 'Chrome' : 'ChromeHeadless'],

Let's use headless mode for 11.2 as well so we can use a universal build environment.