[Tests] Fix public bucket tests

[romilbhardwaj]

sky-imagenet-data is no longer public. Changes to an actual public S3 bucket.

• pytest tests/test_smoke.py::test_managed_jobs_storage --aws

[Michaelvll]

A future TODO: we need to add a test for testing the access to a user's private bucket on the remote cluster.

[yika-luo]

Thanks!

[romilbhardwaj]

A future TODO: we need to add a test for testing the access to a user's private bucket on the remote cluster.

For regular sky launch, this should be covered in the test_<cloud>_storage_mounts_with_stop series of tests. For managed jobs we test COPY mode mounting of private buckets in test_managed_jobs_storage, but not MOUNT mode...