Fix CI for pull requests

[paulo-ferraz-oliveira]

Description

It seems pull_request is required for CI to run when not pushing to the repo itself.

• [x] I have performed a self-review of my changes
• [x] I have read and understood the contributing guidelines

[paulo-ferraz-oliveira]

@starbelly, I'm merging this without a review, since:

1. it's blocking a pull request
2. a previous pull request to remove this had been approved without me thinking of the consequences