Configure `TrustAnchorFile` in OpenDKIM config

[link2xt]

This is needed for DNSSEC support. In main chatmail repository it is configured. TrustAnchorFile is taken from unbound, maybe OpenDKIM should be compiled with unbound like we already modify OpenDKIM package to compile with lua:

nixos-chatmail/overlay.nix

Line 5 in cdc84da

configureFlags = old.configureFlags ++ [ "--with-lua=${prev.lua}" ];