Trigger CI on PRs from forks

[adityagoel4512]

No description provided.

[neNasko1]

I think we should also change the "ref" attribute to the default?

The actions README says:

    # The branch, tag or SHA to checkout. When checking out the repository that
    # triggered a workflow, this defaults to the reference or SHA for that event.
    # Otherwise, uses the default branch.
    ref: ''

Keeping it the same caused problems for me in spox.