nixos: permit dnscrypt-proxy service to read basic user/group info

If nscd is not running, dnscrypt-proxy crashes without read access
to /etc/{password,group,nsswitch.conf}.

nixos/modules/services/networking/dnscrypt-proxy.nix

···
           /dev/null rw,
           /dev/urandom r,
 
+          /etc/passwd r,
+          /etc/group r,
+          ${config.environment.etc."nsswitch.conf".source} r,
+
           ${pkgs.glibc}/lib/*.so mr,
           ${pkgs.tzdata}/share/zoneinfo/** r,