Fix nightly pipeline: install uv with python 3.12, resilient s3 ls

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.buildkite/nightly-release-pipeline.yaml

@@ -4,6 +4,9 @@ steps:
     agents:
       queue: cpu_queue_release
     commands:
+      - "curl -LsSf https://astral.sh/uv/install.sh | sh"
+      - 'export PATH="$HOME/.local/bin:$PATH"'
+      - "uv venv --python=3.12 && source .venv/bin/activate"
       - "uv pip install --upgrade build"
       - "python3 -m build"
       - "bash .buildkite/scripts/upload-nightly-wheels.sh"

.buildkite/scripts/generate-and-upload-nightly-index.sh

@@ -29,7 +29,7 @@ echo "Python version: $($PYTHON --version)"
 
 # list all wheels in the commit directory
 echo "Existing wheels on S3:"
-aws s3 ls "$S3_COMMIT_PREFIX"
+aws s3 ls "$S3_COMMIT_PREFIX" || echo "(no objects found)"
 obj_json="objects.json"
 aws s3api list-objects-v2 --bucket "$BUCKET" --prefix "$SUBPATH/" --delimiter / --output json > "$obj_json"
 mkdir -p "$INDICES_OUTPUT_DIR"