Checkout PR ref

Author: sarahchen6

.github/workflows/create-test-mirror-pr.yml

@@ -25,6 +25,7 @@ jobs:
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           path: dd-trace-java-docker-build
+          ref: refs/pull/${{ github.event.inputs.pr_number }}/merge
 
       - name: Checkout DataDog/images
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0