Disable deployment workflow in forks

[ctmbl]

see https://github.com/ctmbl/blog.iscsc.fr/actions workflows are automatically running on up-to-date fork's main branch, but they will always fail at the deployment step because of missing secrets

We'd like to disable deployment and notifying for forks

the subject has been discussed already: github/docs#6861 (comment)

Note
if the report is still there here is the error I'm dealing with: https://github.com/ctmbl/blog.iscsc.fr/actions/runs/8126377796