Allow managed downloads with --python-preference system (#8808)

Closes https://github.com/astral-sh/uv/issues/8804

crates/uv-python/src/discovery.rs

@@ -1546,7 +1546,10 @@ impl PythonPreference {
     }
 
     pub(crate) fn allows_managed(self) -> bool {
-        matches!(self, Self::Managed | Self::OnlyManaged)
+        match self {
+            Self::OnlySystem => false,
+            Self::Managed | Self::System | Self::OnlyManaged => true,
+        }
     }
 }