Fix bug rendering group icons when forking

- NamespacesHelper#namespace_icon should return a URL instead of an image_tag for groups

app/helpers/namespaces_helper.rb

@@ -31,7 +31,7 @@ module NamespacesHelper
 
   def namespace_icon(namespace, size = 40)
     if namespace.is_a?(Group)
-      group_icon(namespace)
+      group_icon_url(namespace)
     else
       avatar_icon_for_user(namespace.owner, size)
     end

changelogs/unreleased/44508-fix-fork-namespace-images.yml

@@ -0,0 +1,5 @@
+---
+title: Fix bug rendering group icons when forking
+merge_request:
+author:
+type: fixed