sway: Set XDG_CURRENT_DESKTOP=sway

This seems like a good idea in general and will at least make it easier
to get screen sharing to work (but still requires a proper NixOS
configuration including xdg-desktop-portal-wlr).

Note: It isn't necessary to set XDG_SESSION_TYPE=wayland as wlroots
already takes care of it (currently at least for the logind and libseat
session backends, the next wlroots release will require libseat):
4839664a92/backend/session/session.c (L80)

pkgs/applications/window-managers/sway/wrapper.nix

@@ -14,6 +14,7 @@ let
   baseWrapper = writeShellScriptBin "sway" ''
      set -o errexit
      if [ ! "$_SWAY_WRAPPER_ALREADY_EXECUTED" ]; then
+       export XDG_CURRENT_DESKTOP=sway
        ${extraSessionCommands}
        export _SWAY_WRAPPER_ALREADY_EXECUTED=1
      fi