Use astral.sh instead of example.com in lock_unique_named_index (#14657)

This test flakes a lot, maybe using a different domain will help

Closes https://github.com/astral-sh/uv/issues/14542

crates/uv/tests/it/lock.rs

@@ -16412,7 +16412,7 @@ fn lock_unique_named_index() -> Result<()> {
 
         [[tool.uv.index]]
         name = "example"
-        url = "https://example.com"
+        url = "https://astral.sh"
         "#,
     )?;