riot-desktop: fix missing tray icon

The tray icon has always been missing for me (on sway / waybar).
It turns out that it is looked up in a different location than where we package it.

Overall, it seems we are not at all following the upstream layout as can
be seen in data.tar.xz available at packages.riot.im/debian/pool/main/r/riot-web/riot-web_1.4.2_amd64.deb

This is a small fix that works for me. May not be optimal :-).

pkgs/applications/networking/instant-messengers/riot/riot-desktop.nix

@@ -31,6 +31,7 @@ in yarn2nix-moretea.mkYarnPackage rec {
     ln -s '${riot-web}' "$out/share/riot/webapp"
     cp -r '${riot-web-src}/origin_migrator' "$out/share/riot/origin_migrator"
     cp -r './deps/riot-web' "$out/share/riot/electron"
+    cp -r './deps/riot-web/img' "$out/share/riot"
     rm "$out/share/riot/electron/node_modules"
     cp -r './node_modules' "$out/share/riot/electron"