nixos/ntpd: automagically use pool instead of server (#451296)

nixos/modules/services/networking/ntp/ntpd.nix

···
     restrict 127.0.0.1
     restrict -6 ::1
 
-    ${toString (map (server: "server " + server + " iburst\n") cfg.servers)}
+    ${toString (
+      map (
+        server: "${if lib.strings.hasInfix "pool" server then "pool" else "server"} " + server + " iburst\n"
+      ) cfg.servers
+    )}
 
     ${cfg.extraConfig}
   '';