Preview Workflow: Use pull_request_target to avoid token restriction

[vcanales]

Due to restrictions imposed to the GH_TOKEN when forks are created from forks when workflows are triggered by the pull_request event, I'm switching the execution to be done on pull_request_trigger, which lifts said restrictions.

Docs: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

This event runs in the context of the base of the pull request, rather than in the context of the merge commit, as the pull_request event does. This prevents execution of unsafe code from the head of the pull request that could alter your repository or steal any secrets you use in your workflow. This event allows your workflow to do things like label or comment on pull requests from forks. Avoid using this event if you need to build or run code from the pull request.