Quarantine flaky performance bar test

qa/qa/specs/features/browser_ui/non_devops/performance_bar_spec.rb

 # frozen_string_literal: true
 
 module QA
-  context 'Performance bar' do
+  # https://gitlab.com/gitlab-org/gitlab/issues/38315
+  context 'Performance bar', :quarantine do
     context 'when logged in as an admin user', :requires_admin do
       before do
         Flow::Login.sign_in_as_admin