Disable dask-expr in docs builds. (#15343)

Fixes CI blocked by dask-expr.

xref:
- #14805
- rapidsai/rapids-dask-dependency#33

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Ray Douglass (https://github.com/raydouglass)
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #15343

ci/build_docs.sh

@@ -41,6 +41,9 @@ mkdir -p "${RAPIDS_DOCS_DIR}/libcudf/html"
 mv html/* "${RAPIDS_DOCS_DIR}/libcudf/html"
 popd
 
+# TODO: Remove this once dask-expr works in the 10min notebook
+export DASK_DATAFRAME__QUERY_PLANNING=False
+
 rapids-logger "Build Python docs"
 pushd docs/cudf
 make dirhtml