Adds a new system test, test_basic_wrd_x_region, to verify functionality with cross-region GCS buckets.

Also updates the Cloud Build configuration to pass the necessary _CROSS_REGION_BUCKET environment variable to the test environment.

Renames open_retries to _open_retries in AsyncMultiRangeDownloader to encapsulate the retry counter. Updates test_zonal.py to verify the private attribute.