ENH Update allow for env var config of gpuci_conda_retry

[mike-wendt]

Allow gpuci_conda_retry to be configured by env vars as well as flags. This brings it inline with how gpuci_retry operates.

[mike-wendt]

Allowing env var settings allows us to fix build errors seen in NVIDIA/cudf#6329 when gpuci_retry with conda was used. With the env var settings this simplifies the use and prevents the build errors seen:

gpuci_logger "Build conda pkg for libcudf"
gpuci_conda_retry build conda/recipes/libcudf

gpuci_logger "Build conda pkg for libcudf_kafka"
gpuci_conda_retry build conda/recipes/libcudf_kafka

gpuci_logger "Build conda pkg for cudf"
gpuci_conda_retry build conda/recipes/cudf --python=$PYTHON

gpuci_logger "Build conda pkg for dask-cudf"
gpuci_conda_retry build conda/recipes/dask-cudf --python=$PYTHON

gpuci_logger "Build conda pkg for cudf_kafka"
gpuci_conda_retry build conda/recipes/cudf_kafka --python=$PYTHON

gpuci_logger "Build conda pkg for custreamz"
gpuci_conda_retry build conda/recipes/custreamz --python=$PYTHON