Add retry configuration to conda setup steps in CI
The conda3.8 on windows job failed with a transient network error
(IncompleteRead) while downloading packages during setup-miniconda.
Configure conda's built-in retry mechanism via environment variables:
- CONDA_REMOTE_MAX_RETRIES=5 (up from default 3)
- CONDA_REMOTE_BACKOFF_FACTOR=2 (exponential backoff)
