fix: GitHub Action can now be used on ubuntu-18.04 runner

[mayeut]

Always install nox with python3.10 in order to allow using the action with an ubuntu-18.04 runner.

[FollowTheProcess]

It might be a good idea (given the number of jobs that will be run here) to put a condition in the CI workflow to only run the actions tests if action.yml has changed?

Just so every PR isn't clobbered with a load of actions tests if they've not changed it

Something like this: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-including-paths

[mayeut]

It might be a good idea (given the number of jobs that will be run here) to put a condition in the CI workflow to only run the actions tests if action.yml has changed?

That makes sense. I'd say only run on push to main or when there are changes in the action.

I can either create a new workflow file to use built-in filtering as you mentioned or introduce a new workflow dependency https://github.com/dorny/paths-filter to keep only one workflow file.
Please let me know your preference.

[FollowTheProcess]

I'd say only run on push to main or when there are changes in the action.

Yep sounds good to me!

I think a separate workflow file makes the most sense to me, that way we have one for the main project and one to test the GitHub action

[mayeut]

I'd say only run on push to main or when there are changes in the action.

Well, GitHub doesn't allow to do an "OR filter" for push so I used main and **action** for branch name on push.

[FollowTheProcess]

Thanks @mayeut 🎉