Fix pip in compat helper workflow

.github/workflows/update_backend.yml

@@ -9,6 +9,9 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4
+        with:
+          python-version: 3.9
+          cache: pip
 
       - name: "Install PySR"
         run: |