ARROW-7729: [Python][CI] Pin pandas version to 0.25 in the dask integ…

…ration test

Additionally test agains dask's latest release not just the master revision.

Closes #6326 from kszucs/dask-pandas-pin and squashes the following commits:

b5cb40e <Krisztián Szűcs> pin pandas depending on dask's version
083221c <Krisztián Szűcs> pin pandas in the dask integration test

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>

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 if [ ${dask} == "latest" ]; then conda install pandas=0.25.3; fi