Fix pip update for windows workflow (#1502)

Author: echarlaix

.github/workflows/test_openvino_slow.yml

@@ -58,7 +58,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-          pip install --upgrade pip uv
+          python -m pip install --upgrade pip uv
           uv pip install .[openvino,tests,diffusers] transformers[testing]
 
       - if: ${{ matrix.transformers-version != 'latest' && matrix.transformers-version != 'main' }}