ci: Run on push only in the main branch

[vigneshvg]

This avoids excessive CI runs when pushing changes to other
branches on forks.

[wantehchang]

Vignesh: Thanks for the improvement. I am not familiar with the "run on" settings, so I am deferring to Vincent and Yannis.

We need to make sure we run these on release branches. Is there a way to specify that?

We can assume our release branches follow the naming convension v*.*.x.

[y-guyon]

I disabled GitHub actions on my fork some time ago to avoid this issue. Local tests and PR tests are enough for me.

We need to make sure we run these on release branches.

I agree. Maybe restricting to base repository branches would be better but that may be difficult to setup.

[vrabaud]

I agree, we should keep that for release branches and tags:

branches
  - main
  - 'v*.*.x'
tags
  - v*.*.x

cf https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-including-branches-and-tags