[Security]: enable CI (GitHub Actions) on external contributions

[ldez]

#627 (comment)

Our process prioritizes validating external code before CI runs.

Currently, you never run CI on external contributions.
So that means after reviewing a PR you run it locally on your computer.
I think that is a security problem more than using GitHub Action on PR from the forks.

The run of the GitHub Actions is controlled by repository settings:

https://github.com/exoscale/egoscale/settings/actions

You should check this option "Require approval for all outside collaborators" and the CI will only run after a CI approval (not a PR approval).

Also, the secrets are never used with a PR from forks, so using forks instead of branches is a lot more secure.
Creating PR from a branch of a repository allows access to secrets, this is a security problem.

I think you should update your security practices and policies.

Related to #627