The necessary steps have in fact been documented here: https://github.com/arduino/report-size-deltas/blob/main/docs/FAQ.md#workflow-triggered-by-pull_request-event but I have overlooked them when I fixed the upload issue. With this PR the size deltas are - once again - reported within the PR.
Arduino Low Power library
This library allows the use of the low power features of the SAMD21 MCU. This means your battery powered projects will have a longer battery life on boards like MKRZero, MKR1000 and MKRFox1200.
For more information about this library please visit us at
https://www.arduino.cc/en/Reference/ArduinoLowPower
License
Copyright (c) Arduino LLC. All right reserved.
This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA