astroid: fix crash due to missing icons

Astroid was unable to run in a pure environment because it couldn't find
the necessary icons. This fixes that issue, and was tested with
`nix-shell --pure`

pkgs/applications/networking/mailreaders/astroid/default.nix

···
 
   preFixup = ''
     wrapProgram "$out/bin/astroid" \
-      --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH"
+      --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH:${gnome3.adwaita-icon-theme}/share"
   '';
 
   meta = {