If the USE_PLAYBIN3=1 env var is set, we want to replace
playbin with playbin3, but separate to that, we always
want to register the 'playbin3' element so that applications
which explicitly use playbin3 work regardless of the env var.
This fixes `USE_PLAYBIN3=1 gst-validate-launcher`, for example.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1102>
e06ac494b8