Ensure that PR previews also deploy on forked repositories

[Zethson]

See e.g .https://github.com/theislab/single-cell-best-practices/actions/runs/14578921423/job/40891103457?pr=358

gives a permission denied error. Secrets are not available to forks. Need to figure out why this doesn't work for forks here.

[LuisHeinzlmeier]

This is a known problem, and due to security reasons. See, for example, here. Is it important that this also works with forked repos? Then we may have to come up with a new option.

[Zethson]

Yup! Then I guess we might need netlify after all. There are also other workaround mentioned that require maintainer approval which I like. We can revisit this topic at a later point.