As long as you have this commit of core, then in theory pnpm 10.x should automatically drop back to pnpm 9 when run in core. You shouldn’t need any custom install technique.
Immediately after the pnpm 10 release, some team members did have problems, and were able to resolve by running pnpm self-update 9. But since we added that new config to core, I don’t think anyone has needed to do anything manually.
Using VSCode? If you’re using the predefined tasks, then you can add env in tasks.json. If you’re launching rails/ember via the VSCode terminal, then you can add the env as a prefix like any other terminal.
That’s a new one for me 
. Glad you managed to solve it though!
I’m curious, what linux OS are you running?