ci: run ci for all PRs, not only those targeting main

[roobre]

Description

Sometimes it is useful to stack PRs to make reviewing them easier, specially as Github is kind enough to automatically repoint them to main when the base PR gets merged.

Currently, PRs not pointing to main do not get tested. This PR proposes changing that to allow for this kind of workflow.

[pablochacin]

Sometimes it is useful to stack PRs to make reviewing them easier

I don't oppose to enable this feature, but I'm not sure I follow the reasoning of the justification for this change: "reviewing them easier". I'm generally not in favor of stacking PR for review, but only for developers to start follow-up work (e.g. draft a PR) while one PR is still in review. Could you please elaborate on this point?

[roobre]

reviewing them easier

By this I mean that the diff is easier to look at, specially for self-review.

I agree that I don't see any use case for merging one PR into another, but I think it does help for visualizing the changes.

[pablochacin]

I'm still unconvinced opening PR against other branches is a good practice in the context of this project. However, I don't see any obvious risks or drawbacks.

[roobre]

So far I have only found this need twice, so perhaps it's not worth changing. I can close this for now and bring it up again if it becomes an issue again.