I received help from @Peilonrayz, who was very helpful.

The fix for me was to go to the "regular" Windows PowerShell .exe and run

mkdir tmp; cd tmp; python -m venv venv; . venv/Scripts/Activate.ps1; pip install labelme; labelme --help

then

code venv/Lib/site-packages/labelme/__init__.py

which should open the __init__.py file in your editor. Add import onnxruntime at the very top of the file and run labelme in the PowerShell window and it should open the GUI.