pin pandas in the dask integration test

Author: kszucs

ci/docker/conda-python-dask.dockerfile

@@ -23,3 +23,6 @@ FROM ${repo}:${arch}-conda-python-${python}
 ARG dask=latest
 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