librem5-gst: Use playbin3 in webkit

This helps hw accelerated video.

Note that `USE_PLAYBIN3` confuses webkit so use this webkit
specific env var.
3 jobs for gst-gl in 7 minutes and 19 seconds (queued for 8 minutes and 18 seconds)
Status Job ID Name Coverage
  Package
passed #331883
librem5
package:deb-debian

00:01:05

 
  Test Package
passed #331884
librem5
autopkgtest-debian-package

00:05:20

passed #331885
librem5
lintian-debian-package

00:00:52