nixos/kresd: use DNSSEC root trust anchor from nixpkgs

in read-only way.  If the cache directory is empty and you use the
very same service for system's DNS, kresd is unable to bootstrap root
trust anchors, as it would need a DNS lookup.

Also, if we don't rely on bootstrap, the extra lua deps of kresd could
be dropped by default, but let's not do that now, as the difference in
closure size is only ~4 MB, and there may be other use cases than
running the package as nixos service this way.

nixos/modules/services/networking/kresd.nix

@@ -99,7 +99,7 @@ in
 
       script = ''
         exec '${package}/bin/kresd' --config '${configFile}' \
-          -k '${cfg.cacheDir}/root.key'
+          -k '${pkgs.dns-root-data}/root.key'
       '';
 
       requires = [ "kresd.socket" ];