By default, new mpegts output is used; but to allow CI on linux not
to be broken, we allow use of old mpegts output.
Up to ubuntu 22.04 there is no librist package available.
A manual compile is then required but the CI scripts would need to be
updated.
This also allows easy fallback in case of fatal bugs in new output.
Signed-off-by: pkv <pkv@obsproject.com>