[Performance] Use new Azure custom images

[yika-luo]

Use the new custom image for Azure's VM creation.

Example performance change (with fixed region):

VM Type 💻	Old Provision 🕐	New Provision 🕐	% Speedup ✅
Azure GPU	4min	2min 40s	35% (1.5x)
Azure CPU	4min	2min	50% (2x)

Example benchmark commands:
CPU: time sky launch --cloud=azure --region=centralus -y
GPU: time sky launch --cloud=azure --region=centralus -y --gpus=A100-80GB:1

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Relevant individual smoke tests:
  pytest tests/test_smoke.py::test_minimal --azure
pytest tests/test_smoke.py::test_huggingface --azure
pytest tests/test_smoke.py::test_job_queue_with_docker --azure
pytest tests/test_smoke.py::test_cancel_azure

[Michaelvll]

LGTM! Thanks @yika-luo!