Use pull_request_target trigger for CI

[emmyoop]

Describe the bug

PRs from forks will fail CI because the PR trigger is pull_request so they do not have access to the repo vars that are needed since the workflow runs in the context of the fork.

Steps to reproduce

Open a PR from a fork.

Expected results

CI passes

Actual results

CI fails because vars are missing

Additional context

Are you interested in contributing the fix?

yes

[dbeatty10]

@emmyoop Assuming we'll need something similar in dbt-codegen here, dbt-project-evaluator, etc?