pin pandas depending on dask's version

Author: kszucs

ci/docker/conda-python-dask.dockerfile

@@ -25,4 +25,4 @@ COPY ci/scripts/install_dask.sh /arrow/ci/scripts/
 RUN /arrow/ci/scripts/install_dask.sh ${dask}
 
 # The Spark tests currently break with pandas >= 1.0
-RUN conda install pandas=0.25.3
+RUN if [ ${dask} == "latest" ]; then conda install pandas=0.25.3; fi