ci: CI always fails for users who offer PRs

[tico24]

It seems that the ci ghcr creds aren't available to users who offer PRs to this project. They are only available to cool people like myself and @Joibel.

Other github projects don't have this issue, so we've done some incorrect setup that I don't yet understand.

[clementnuss]

just as a side note, it's probably not a good idea to automatically grant access to GITHUB_TOKEN to cool external people (like me): it's then quite simple to create a PR to extract the token and do nasty thing 😈

some more info can be found here

[Joibel]

Indeed. We don't auto run PRs from non-cool people, but it's not really good practice.