[Tests] Fix storage mount smoke test for k8s

[Michaelvll]

When running the test with k8s, the private mount store will be set to Azure causing the issue with fuse3. We now explicitly set the store to s3 when it is on k8s.

For s3 another issue is that pip3 may not be available during the first file mount, so we change to uv pip instead.

To reproduce:
pytest tests/test_smoke.py::test_docker_storage_mounts[docker:ubuntu:18.04] --kubernetes
and adding the following to

skypilot/sky/task.py

Lines 941 to 946 in 83b2325

	if storage_cloud is None:
	storage_cloud_str = enabled_storage_clouds[0]
	assert storage_cloud_str is not None, enabled_storage_clouds[0]
	storage_region = None # Use default region in the Store class
	else:
	storage_cloud_str = str(storage_cloud)

        # TODO(zhwu): debugging only
        if storage_cloud is None:
            # TODO(zhwu): debugging only
            if 'azure' in enabled_storage_clouds:
                storage_cloud_str = 'azure'
            else:
                storage_cloud_str = enabled_storage_clouds[0]

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
  • pytest tests/test_smoke.py::test_docker_storage_mounts[docker:ubuntu:18.04]
  • pytest tests/test_smoke.py::test_docker_storage_mounts[docker:ubuntu:18.04] --kubernetes
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[romilbhardwaj]

IIUC this also forces s3 when testing on Azure, which would require AWS credentials even if we want to run just Azure tests. Should we retain the original behavior of excluding the private mount if Azure is specified?