UX when running on PR from fork

[lidel]

When running on PR made from form, secrets will not be present, and build will fail.

Example: https://github.com/ipfs/ipfs-docs/actions/runs/14136279402/job/39608667797?pr=1905
Example: https://github.com/ipfs/specs/actions/runs/15367277509/job/43344278503?pr=505

I think we could improve things and detect when running on PR from fork and when secrets are not present.
E.g. don't do pinning, but link to a CAR attached to the build?

Other ideas welcome.

[2color]

I recently updated the workflow in the docs to allow forks access: ipfs/ipfs-docs@54f52c4

There are some obvious risks to using pull_request_target, but those can be reduced by adding restrictions in the repo config on when fork builds run.

Either way, we should still add a check to see if it's running in a fork and log accordingly.