Update test_qlib_from_source_slow.yml

.github/workflows/test_qlib_from_source_slow.yml

@@ -29,7 +29,9 @@ jobs:
 
     - name: Set up Python tools
       run: |
-        pip install --upgrade cython numpy pip
+        python -m pip install --upgrade pip
+        # python -m pip is necessary to upgrade pip.
+        pip install --upgrade cython numpy
         pip install -e .[dev]
 
     - name: Downloads dependencies data