x11-wm/niri: set XDG_SESSION_TYPE by default (a la niri-session)

Smithay (unlike wlroots) doesn't seem to set XDG_SESSION_TYPE.
Mainly affects Qt which prefers XDG_SESSION_TYPE over WAYLAND_DISPLAY.

x11-wm/niri/files/patch-no-systemd

@@ -42,12 +42,13 @@ Avoid various systemd dependencies
      let cli = Cli::parse();
 
      let _client = tracy_client::Client::start();
-@@ -128,6 +114,9 @@ fn main() {
+@@ -128,6 +114,10 @@ fn main() {
          socket_name.to_string_lossy()
      );
 
 +    // Advise xdg-desktop-portal which *-portals.conf to load
 +    env::set_var("XDG_CURRENT_DESKTOP", "niri");
++    env::set_var("XDG_SESSION_TYPE", "wayland");
 +
      if is_systemd_service {
          // We're starting as a systemd service. Export our variables.