fix: fixing perf test workflow during merge. (#16574)

.github/workflows/test-build-docker-image.yml

@@ -430,11 +430,15 @@ jobs:
         ports:
           - 27017:27017
     steps:
-      # Check out merge commitGIT BRANCH
-      - name: Fork based /ok-to-test-perf checkout
+      - name: Checkout the merged commit from PR and base branch
+        if: github.event_name == 'pull_request_review'
         uses: actions/checkout@v2
         with:
-          ref: "refs/pull/${{ github.event.pull_request.number }}/merge"
+          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+
+      - name: Checkout the head commit of the branch
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        uses: actions/checkout@v2
 
       # Timestamp will be used to create cache key
       - id: timestamp