Fix a crash when an Executor wrapped fork exit.

Fix rails#51298

forking inside an Execution Wrapper crashes when running the completion callbacks.

Rails 7.0 was not Execution wrapping the `runner` command.
Rails 7.2 changed the definition of `active_connection?`, the new definition doesn't contain the bug.

Therefore forking inside a script intended to be run with the `runner` command on 7.1 crashes. (see rails#51298)

activerecord/lib/active_record/query_cache.rb

@@ -30,7 +30,7 @@ def self.run
     end
 
     def self.complete(pools)
-      pools.each { |pool| pool.disable_query_cache! }
+      pools.each { |pool| pool.disable_query_cache! unless pool.discarded? }
 
       ActiveRecord::Base.connection_handler.each_connection_pool do |pool|
         pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?