chore: fix preview release to use correct target (#1104)

## What kind of change does this PR introduce?

Change event trigger from `pull_request` to `pull_request_target`.

## What is the current behavior?

Event for triggering a PR release is `pull_request`. This results in PRs
from forks not being able to trigger the cross-repo testing.

## What is the new behavior?

Event trigger is now `pull_request_target`. Using `pull_request_target`
allows access to secrets when PRs come from forks. This is secure, since
only us, maintainers, can add the label to trigger the PR release.