check out the source repository on PRs (#189)

Author: chrisfenner

.github/workflows/render.yml

@@ -48,12 +48,13 @@ jobs:
           echo OUTPUT_FILENAME="${filename}" >> "$GITHUB_OUTPUT"
           echo output filename: ${filename}
 
-      # For pull requests, check out the head ref of the source branch.
+      # For pull requests, check out the head ref of the source branch from the source repo.
       - name: Checkout
         if: inputs.workflow == 'pr'
         uses: actions/checkout@v4
         with:
           ref: ${{ github.head_ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
           fetch-depth: 0
           fetch-tags: true
       # For other workflows, check out the requested revision (which defaults to