Fix race in forbid_sidekiq_in_transactions.rb

What does this MR do?

Uses Thread.current instead of mattr_accessor.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64088

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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