netdata module: fix ExecStartPost (#181976)

nixos/modules/services/monitoring/netdata.nix

···
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/netdata -P /run/netdata/netdata.pid -D -c /etc/netdata/netdata.conf";
         ExecReload = "${pkgs.util-linux}/bin/kill -s HUP -s USR1 -s USR2 $MAINPID";
-        ExecStartPost = ''while [ "$(netdatacli ping)" != pong ]; do sleep 0.5; done'';
+        ExecStartPost = pkgs.writeShellScript "wait-for-netdata-up" ''
+          while [ "$(${pkgs.netdata}/bin/netdatacli ping)" != pong ]; do sleep 0.5; done
+        '';
 
         TimeoutStopSec = 60;
         Restart = "on-failure";