nixos/nsd: symlink conf file to /etc/nsd

We remove the configFile build flag override in the NixOS module.

Instead of embedding the conf file link to the binaries, we symlink it
to /etc/nsd/nsd.nix, the hardcoded config file location for the
various CLI nsd utilities.

This config file build option override is triggerring a nsd rebuild
for each configuration change. This prevent us to use the nixos cache
in many cases.

Co-authored-by: Erjo <erjo@cocoba.work>