Forbid the name attribute for fetchTree

We need to support it for the “old” fetch* functions for backwards
compatibility, but we don’t need it for fetchTree (as it’s a new
function).
Given that changing the `name` messes-up the content hashing, we can
just forbid passing a custom `name` argument to it

src/libexpr/primops/fetchTree.cc

@@ -119,6 +119,13 @@ static void fetchTree(
                 .errPos = pos
             });
 
+        if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
+            throw Error({
+                .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
+                .errPos = pos
+            });
+
+
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
         auto url = fixURI(state.coerceToString(pos, *args[0], context, false, false), state);