Change trigger to pull_request_target

And then switch to PR code to utilize secrets from main repository

Author: romanowski

.github/workflows/scala3doc.yaml

@@ -4,7 +4,7 @@ on:
   push:
     branches:
       - master
-  pull_request:
+  pull_request_target:
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -13,6 +13,19 @@ jobs:
       - name: Git Checkout
         uses: actions/checkout@v2
 
+      - name: Checkout to PR code
+        env:
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run : |
+          if [[ -z "${PR_NUMBER}" ]]; then
+            REF=refs/pull/$PR_NUMBER/merge
+            echo checking $REF
+            git fetch origin $REF:$REF
+            git checkout $REF
+          else
+            echo Not a pull request do not need to checkout
+          fi     
+
       - name: Cache Coursier
         uses: actions/cache@v1
         with: