Use strict priority in CI conda tests (#819)

## Description
This PR sets conda to use `strict` priority in CI tests.

Mixing channel priority is frequently a cause of unexpected errors. Our
CI jobs should always use strict priority in order to enforce that conda
packages come from local channels with the artifacts built in CI, not
mixing with older nightly artifacts from the `rapidsai-nightly` channel
or other sources.

xref: rapidsai/build-planning#14

---------

Co-authored-by: Gil Forsyth <[email protected]>

Author: bdice

ci/check_conda_nightly_env.py

@@ -4,7 +4,6 @@
 import sys
 from datetime import datetime, timedelta
 
-
 OLD_PACKAGE_THRESHOLD_DAYS = 3
 
 EXCLUDED_PACKAGES = {

ci/test_conda_nightly_env.sh

@@ -8,6 +8,9 @@ CUDA_VERSION=${RAPIDS_CUDA_VERSION%.*}
 
 JSON_FILENAME="rapids_cuda${CUDA_VERSION}_py${RAPIDS_PY_VERSION}.json"
 
+rapids-logger "Configuring conda strict channel priority"
+conda config --set channel_priority strict
+
 rapids-logger "Creating conda environment with rapids=${RAPIDS_VERSION}, python=${RAPIDS_PY_VERSION}, cuda-version=${CUDA_VERSION}"
 
 rapids-conda-retry \