ci: fix checkout for PRs from forks

Use head repo and SHA when checking out pull_request events so the
workflow can fetch the branch from the fork instead of the base repo.
Fixes CI failure when patch-1 (or other fork branches) don't exist
in the base repository.

Co-authored-by: Ali Khokhar <alishahryar2@gmail.com>

.github/workflows/tests.yml

@@ -14,7 +14,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          ref: ${{ github.head_ref || github.ref_name }}
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name || github.repository }}
           fetch-depth: 1
  
       - name: "Fail on type: ignore (no suppressions allowed)"