Add bicep format to CI

.github/workflows/azure-bicep-validate.yaml

@@ -13,8 +13,17 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+
     steps:
-      - name: Checkout
+      - name: Checkout PR
+        if: ${{ github.event_name == 'pull_request'}}
+        uses: actions/checkout@v3
+        with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.ref }}
+
+      - name: Checkout branch
+        if: ${{ github.event_name == 'push'}}
         uses: actions/checkout@v2
 
       - name: Build Bicep