gnu: ungoogled-chromium: Build with ffmpeg@4.2.

Fixes <https://bugs.gnu.org/41987>.
Reported by Jonathan Brielmaier <jonathan.brielmaier@web.de>.

* gnu/packages/video.scm (ffmpeg-4.2): New public variable.
* gnu/packages/chromium.scm (ungoogled-chromium)[inputs]: Change from FFMPEG
to FFMPEG-4.2.