ci: server: fix python installation (ggml-org#6918)

Author: phymbert

.github/workflows/server.yml

@@ -100,6 +100,12 @@ jobs:
               -DLLAMA_SANITIZE_${{ matrix.sanitizer }}=ON ;
           cmake --build . --config ${{ matrix.build_type }} -j $(nproc) --target server
 
+      - name: Python setup
+        id: setup_python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
       - name: Tests dependencies
         id: test_dependencies
         run: |