[wip] improved github actions workflow
This is a WIP work on https://github.com/huggingface/transformers/issues/7887
@sshleifer, I don't think I can do anything here w/o repo permissions. I have no access to the runners.

https://github.com/stas00/transformers/actions/runs/319165633

> No runner matching the specified labels was found: self-hosted, multi-gpu<|||||>github is so borked! It now won't let me delete the new workflow - and keeps running it automatically and emailing me on each commit I make elsewhere in this project - what horrors!<|||||>ask on https://github.community/, they are pretty responsive.