dgraph: use --prefix to prefix PATH

This is more robust in case PATH is empty or otherwise weird.

nixos/modules/services/databases/dgraph.nix

@@ -12,7 +12,7 @@ let
   ''
     mkdir -p $out/bin
     makeWrapper ${cfg.package}/bin/dgraph $out/bin/dgraph \
-      --set PATH '${lib.makeBinPath [ pkgs.nodejs ]}:$PATH' \
+      --prefix PATH : "${lib.makeBinPath [ pkgs.nodejs ]}" \
   '';
   securityOptions = {
       NoNewPrivileges = true;