Unstick Windows update by resetting network connection

Windows 10 Fast Ring builds keep sticking at 87% before updating. I got into a giant boot loop where the update would fail or I’d hard reboot (hold power button) and this was not working. Finally for lack of things to try, I unplugged the Ethernet connector when it Windows Update was stuck at 87%, then upon plugging in, Windows Update very rapidly finished the update and rebooted and continued installing Windows.

This happened yet again for Build 14986, and once again, unplugging Ethernet (or Ethernet-USB adapter, or undocking/docking) made Windows Update complete rapidly and successfully.