github-ci: fix new authors check

Shortly after minimizing the new authors check to one workflow, github
released an action update that required more permissions due to an
attack scenario, which didn't really apply to us as we didn't run code
from the remotes fork.

However, to avoid extending permissions, rework the authors check to
pull the OISF repo, checkout the PR fork as history, then do the new
authors check. This safely allows the check to run in the context of our
repo without the fork being able to inject code execution.

Author: jasonish

.github/workflows/authors.yml

@@ -4,12 +4,14 @@ name: New Authors Check
 # for fork PRs, allowing us to post a comment directly without the artifact
 # hand-off to a second workflow_run workflow.
 #
-# Only git history is read here — no PR code is built or executed — so
-# checking out the PR head SHA is safe under pull_request_target.
+# Only git history is read here. Avoid checking out fork PR files in the
+# pull_request_target trust context; the job checks out the trusted base ref and
+# fetches the PR head ref only for git-log inspection.
 on:
   pull_request_target:
 
 permissions:
+  contents: read
   pull-requests: write
 
 concurrency:
@@ -21,16 +23,24 @@ jobs:
     name: New Author Check
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout PR code
+      - name: Checkout trusted base branch
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0
         with:
-          ref: ${{ github.event.pull_request.head.sha }}
           fetch-depth: 0
+          persist-credentials: false
       - run: sudo apt -y install git
+      - name: Fetch PR head for history inspection
+        env:
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          git fetch --no-tags --no-recurse-submodules origin \
+            "+refs/pull/${PR_NUMBER}/head:refs/remotes/pull-request/head"
       - name: Export known authors from main branch
         run: git log --format="%an <%ae>" origin/main | sort | uniq > authors.txt
       - name: Export authors from new commits
-        run: git log --format="%an <%ae>" ${{ github.event.pull_request.base.sha }}... | sort | uniq > commit-authors.txt
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: git log --format="%an <%ae>" "${BASE_SHA}..refs/remotes/pull-request/head" | sort | uniq > commit-authors.txt
       - name: Check new authors
         run: |
           touch new-authors.txt