CI: pull_request_target lint/test checks out base branch instead of PR head

[SergioChan]

Summary

codetests.yml runs on pull_request_target, but each job uses actions/checkout without an explicit PR ref/repository.

On fork-based PRs, this checks out upstream main instead of the PR head commit, so lint/test output can report unrelated failures from main.

Reproduction

1. Open a PR from a fork.
2. Trigger test-and-lint (pull_request_target).
3. In logs, observe checkout resolving to origin/main (base branch), not the PR head SHA.

Observed in: #618

Expected

For pull_request_target, checkout should target the PR head repository + SHA so CI validates the actual PR changes.