ci: also unblock pr-security-review fork checkout (author-gated, read-only)

Completes the checkout@v7 fork-PR fix: pr-security-review.yml has the same
pull_request_target + checkout@v7 + fork-head pattern (its ref is computed in a
step, so it was easy to miss). Unlike e2e/pr-tarball, this job:
  - is ALREADY gated on the PR author's write/admin permission
    (getCollaboratorPermissionLevel), so external forks already skip it, and
  - only READS the code (compute diff, build prompt, run the review) — it never
    executes fork code (no npm ci / npm scripts).
So `allow-unsafe-pr-checkout: true` is the correct, low-risk fix here (reviewing the
PR head is the job's whole purpose), whereas for the secret-executing e2e/pr-tarball
jobs we instead moved to author-based gating and dropped the flag.

Author: aidandaly24

.github/workflows/pr-security-review.yml

@@ -160,6 +160,12 @@ jobs:
         with:
           ref: ${{ steps.pr.outputs.head_sha }}
           fetch-depth: 0
+          # Safe to opt in here: the `authorize` job already gates on the PR AUTHOR having write/admin
+          # (getCollaboratorPermissionLevel), and this job only READS the code (compute diff, build
+          # prompt, run the review) — it never executes fork code (no npm ci / npm scripts). checkout@v7
+          # otherwise refuses fork-PR checkout in pull_request_target, which would break the review on
+          # an authorized author's fork PR. Reviewing the PR head is the whole point of this job.
+          allow-unsafe-pr-checkout: true
 
       - name: Compute diff
         id: diff