You can download the Arduino Firmware Uploader directly from the GitHub release page:
https://github.com/arduino/arduino-fwuploader/releases/latest
To build we use task for simplicity. From the sources root directory run:
task dist:<OS>_<ARCH>
Where could be one of: macOS,Windows,Linux. And : 32bit, 64bit, ARM or ARM64
This will create the arduino-fwuploader executable.