[ci] fix: Install mlflow dependency (#3817)

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Fix the CI error introduced for
[gpu_unit_tests](https://github.com/volcengine/verl/actions/runs/18618017625/job/53085287915?pr=3814#logs)
in #3736

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Signed-off-by: Hollow Man <hollowman@opensuse.org>

.github/workflows/gpu_unit_tests.yml

@@ -93,6 +93,7 @@ jobs:
           pip3 install --no-deps -e .[test]
           pip3 install --upgrade "ray>=2.40.0"
           pip3 install cupy-cuda12x
+          pip3 install mlflow
       - name: Download Model to Use
         run: |
           huggingface-cli download Qwen/Qwen2.5-0.5B-Instruct