totoro: teach how to install grafana plugins from nix

ops/nixos/totoro/default.nix

@@ -227,6 +227,19 @@ in {
       security.cookie_secure = "true";
     };
   };
+  systemd.services.grafana.preStart = let
+    cfg = config.services.grafana;
+    plugins = with depot.pkgs.grafana-plugins; [
+      grafana-piechart-panel
+    ];
+    pluginLines = lib.concatMapStringsSep "\n" (pkg: ''
+      ln -sf ${pkg} ${cfg.dataDir}/plugins/${pkg.pname}
+    '') plugins;
+  in lib.mkAfter ''
+    rm -rf ${cfg.dataDir}/plugins
+    mkdir ${cfg.dataDir}/plugins
+    ${pluginLines}
+  '';
 
   system.stateVersion = "20.03";
 }