Steps to Reproduce:

1. Create a pyproject.toml file with the following content:

   [project]
   name = "app"
   version = "0.1.0"
   dependencies = [
       "urllib3",
   ]
   
   [tool.uv]
   package = false
   
   [tool.uv.pip]
   index-url = "https://test.pypi.org/simple"

2. Run the following command:

   ~/app# uv sync --no-cache --upgrade --verbose 2>&1 | grep simple

3. Observe the output.

Expected Behavior:

The output should indicate that uv is using the specified index-url (https://test.pypi.org/simple) to fetch packages.

Actual Behavior:

The output shows that uv is still attempting to access the default PyPI repository:

DEBUG No cache entry for: https://pypi.org/simple/urllib3/

Environment:

uv Platform: Ubuntu 24.04 LTS on WSL2
uv Version: uv 0.4.0