Store FTS index in directory per domain & user to avoid collisions

Previously all the xapian files and logs would be stored in the same
folder for all users. This couid probably lead to weird situations where
all users get the same search results.

mail-server/dovecot.nix

@@ -33,7 +33,7 @@ let
     "maildir:${cfg.mailDirectory}/%d/%n${maildirLayoutAppendix}"
     + (lib.optionalString
         (cfg.fullTextSearch.enable && (cfg.fullTextSearch.indexDir != null))
-        ":INDEX=${cfg.fullTextSearch.indexDir}"
+        ":INDEX=${cfg.fullTextSearch.indexDir}/%d/%n"
       );
 
   postfixCfg = config.services.postfix;