nixos/sway: restore list type of xdg.portal.config.sway.default

This fixes a regression introduced in #348792, where the option type was
changed from list to string. This commit reverts the type back to list.

Fixes #352188
Closes #352193

nixos/modules/programs/wayland/sway.nix

@@ -159,7 +159,7 @@ in
       # https://github.com/emersion/xdg-desktop-portal-wlr/pull/315
       xdg.portal.config.sway = {
         # Use xdg-desktop-portal-gtk for every portal interface...
-        default = "gtk";
+        default = [ "gtk" ];
         # ... except for the ScreenCast, Screenshot and Secret
         "org.freedesktop.impl.portal.ScreenCast" = "wlr";
         "org.freedesktop.impl.portal.Screenshot" = "wlr";