Build/Test Tools: Use ‘pull_request.head.ref` when checking built files.

Because `pull_request_target` happens in the context of the base branch, attempting to checkout `github.head_ref` results in a failure when the workflow comes from a fork.

This adjusts the options passed to `actions/checkout` to use the repository that actually contains the test branch.

See #62221.

git-svn-id: https://develop.svn.wordpress.org/trunk@60052 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/reusable-check-built-files.yml

@@ -83,7 +83,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ github.head_ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.ref }}
           show-progress: ${{ runner.debug == '1' && 'true' || 'false' }}
           token: ${{ env.ACCESS_TOKEN }}