Think about restricting runs on GitHub Actions

[paulo-ferraz-oliveira]

Per the settings: https://github.com/erlef/otp_builds/settings/actions

We should choose (once we know which ones we'll use) a given number of actions and add those under "Allow erlef, and select non-erlef, actions and reusable workflows".

[paulo-ferraz-oliveira]

Should wait on #1 to understand what is feasible/reasonable. We also need to pin the commit SHAs if we're into security hardening the repo.