don't cache site admin feature_flag_account_ids

Change-Id: I200163a12c002d6cf4e6b3deba597dff4612c81e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232114
Tested-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

lib/feature_flags.rb

@@ -94,6 +94,7 @@ module FeatureFlags
   # starting with site admin
   def feature_flag_account_ids
     return [Account.site_admin.global_id] if is_a?(User)
+    return [] if self.is_a?(Account) && self.site_admin?
 
     RequestCache.cache('feature_flag_account_ids', self) do
       shard.activate do