explicit repository to target another one

+ grant pull request write permission

.github/workflows/diff_user_guides.yml

@@ -1,5 +1,8 @@
 name: Compare generated user guides
 
+permissions:
+  pull-requests: write
+
 on:
   pull_request:
     paths:
@@ -14,6 +17,7 @@ jobs:
       - name: Checkout PR branch
         uses: actions/checkout@v3
         with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }} # PR repository
           ref: ${{ github.head_ref }} # PR branch
 
       - name: Generate files for PR branch
@@ -26,7 +30,8 @@ jobs:
       - name: Checkout target branch
         uses: actions/checkout@v3
         with:
-          ref: ${{ github.base_ref }}
+          repository: ${{ github.event.pull_request.base.repo.full_name }} # Upstream repository
+          ref: ${{ github.base_ref }} # Target branch
 
       - name: Generate files for target branch
         run: |