OK, I've fixed it now. It was quite messy getting it all sorted and I can't remember all the steps.
But rightly or wrongly I uninstalled the nodejs that came with pop!os, then installed vesion 18 nodejs with the official Node.js packages from Nodesource, which are available for Debian-based systems as described at Step 2 (B) here.
I read in an old stackflow thread that to make Yarn available in Ubuntu, start by removing cmdtest and its tools.
After that I ran:
npm cache clean --force
npm install --global yarn
Then the cmd yarn install --cwd vendor/adevtool/ worked as it should and the rest of the build process was ok.
I would mark this thread solved if I could see how!