[r2] CI: fix Python workflow

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

Author: njzjz

.github/workflows/test_python.yml

@@ -34,7 +34,9 @@ jobs:
         # changes, setting `TENSORFLOW_ROOT`.
         TENSORFLOW_VERSION: ${{ matrix.tf }}
         DP_BUILD_TESTING: 1
-    - run: source/install/uv_with_retry.sh pip install --system --no-build-isolation horovod mpi4py
+    - run: |
+        source/install/uv_with_retry.sh pip install --system mpi4py
+        source/install/uv_with_retry.sh pip install --system --no-build-isolation horovod mpi4py
       if: ${{ matrix.tf == '' }}
       env:
         HOROVOD_WITH_TENSORFLOW: 1