Respect `UV_INDEX_` rather than `UV_HTTP_BASIC_` (#8306)
The docs reference `UV_INDEX_`, but the code actually uses
UV_HTTP_BASIC_ as the prefix for environment variable credentials.
See PR #7741
Code is at
https://github.com/astral-sh/uv/blob/main/crates/uv-static/src/env_vars.rs#L163
```rust
/// Generates the environment variable key for the HTTP Basic authentication username.
pub fn http_basic_username(name: &str) -> String {
format!("UV_HTTP_BASIC_{name}_USERNAME")
}
/// Generates the environment variable key for the HTTP Basic authentication password.
pub fn http_basic_password(name: &str) -> String {
format!("UV_HTTP_BASIC_{name}_PASSWORD")
}
```
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
