zigbee2mqtt: enable nixpkgs-update to find update script

pkgs/servers/zigbee2mqtt/default.nix

@@ -14,6 +14,7 @@ package.override rec {
   };
 
   passthru.tests.zigbee2mqtt = nixosTests.zigbee2mqtt;
+  passthru.updateScript = ./update.sh;
 
   postInstall = ''
     npm run build

pkgs/servers/zigbee2mqtt/update.sh

@@ -1,8 +1,11 @@
 #!/usr/bin/env nix-shell
-#! nix-shell -I nixpkgs=../../.. -i bash -p nodePackages.node2nix curl jq nix-update common-updater-scripts
+#! nix-shell -i bash -p nodePackages.node2nix curl jq nix-update common-updater-scripts
 
 set -euo pipefail
 
+SCRIPT_DIR="$(dirname "$0")"
+cd "$SCRIPT_DIR"
+
 CURRENT_VERSION=$(nix eval -f ../../.. --raw zigbee2mqtt.version)
 TARGET_VERSION="$(curl https://api.github.com/repos/Koenkk/zigbee2mqtt/releases/latest | jq -r ".tag_name")"