fix hyper threading ci checkout for external contributors (#1996)

Author: edg-l

.github/workflows/hyper_threading_benchmarks.yml

@@ -11,8 +11,6 @@ jobs:
     steps:
     - name: Checkout PR
       uses: actions/checkout@v4
-      with:
-        ref: ${{ github.head_ref }}
 
     - name: Setup Python
       uses: actions/setup-python@v4