dotfiles/zshrc: set XDG_CURRENT_DESKTOP=sway

This was set in ~/.pam_environment up until a few days ago, but I
forgot to add it when I moved the exports to .zshrc. We need this
for xdg-desktop-portal to know we're on Sway for screensharing.

dotfiles/zshrc

@@ -45,6 +45,8 @@ if [[ "$OSTYPE" =~ ^linux.*$ ]]; then
     # 2020-02-10: https://github.com/swaywm/sway/wiki#issues-with-java-applications
     export _JAVA_AWT_WM_NONREPARENTING=1
     export SDL_VIDEODRIVER=wayland
+    # Signal to xdg-desktop-portal that we're on Sway
+    export XDG_CURRENT_DESKTOP=sway
     {%@@ endif @@%}
 fi