use more explicit ref to upstream repo

Author: bakkot

.github/workflows/pr-preview.yml

@@ -82,7 +82,7 @@ jobs:
 
       - name: Overwrite snapshot scripts with versions from main
         run: |
-          git remote add upstream https://github.com/${{ github.repository }}.git
+          git remote add upstream https://github.com/${{ github.event.pull_request.base.repo.full_name }}.git
           git fetch upstream main
           git checkout upstream/main -- scripts/insert-snapshot-warning.js scripts/deploy-github-pages.sh