Update:
Managed to run big picture with:
let
apps = {
env = "/run/current-system/sw/bin";
apps = [
{
name = "Steam";
output = "steam.txt";
detached = ["${pkgs.util-linux}/bin/setsid ${pkgs.steam}/bin/steam steam://open/bigpicture"];
image-path = "steam.png";
}
];
};
in
{
xdg.configFile = {
"sunshine/sunshine.conf".source = ./sunshine.conf;
"sunshine/apps.json".text = builtins.toJSON apps;
};
}
Still need to figure out why env does not work but this for now does the trick.
Btw, regarding big picture being slow: enable hardware acceleration under settings.