Check out this link
https://www.gamespot.com/articles/how-to-play-fortnite-on-steam-deck/1100-6503402/
Looks like you can stream it from the Xbox service for free via edge, and you can install a flapak version of edge, if your distro can do flapaks
I just did a quick Google, I haven’t actually tried this so good luck! But it sounds like streaming it is the only way given the form of anti cheat used. Don’t think you can do it natively
I use old iPhones as video baby monitors in much the same way with an app called AlfredCamera, works pretty good for a free app on scrap hardware