Fix race in forbid_sidekiq_in_transactions.rb

Current code uses module attribute which stores value
global instead of locally (thread locally). This results
in concurrent accesses to overwrite the each other values

changelogs/unreleased/fix-sidekiq-transaction-check-race.yml

+---
+title: Fix race in forbid_sidekiq_in_transactions.rb
+merge_request: 30359
+author:
+type: fixed

config/initializers/forbid_sidekiq_in_transactions.rb

@@ -2,15 +2,16 @@ module Sidekiq
   module Worker
     EnqueueFromTransactionError = Class.new(StandardError)
-    mattr_accessor :skip_transaction_check
-    self.skip_transaction_check = false
-
     def self.skipping_transaction_check(&block)
-      skip_transaction_check = self.skip_transaction_check
-      self.skip_transaction_check = true
+      previous_skip_transaction_check = self.skip_transaction_check
+      Thread.current[:sidekiq_worker_skip_transaction_check] = true
       yield
     ensure
-      self.skip_transaction_check = skip_transaction_check
+      Thread.current[:sidekiq_worker_skip_transaction_check] = previous_skip_transaction_check
+    end
+
+    def self.skip_transaction_check
+      Thread.current[:sidekiq_worker_skip_transaction_check]
     end
     module ClassMethods