Fix broken master due to hard-coded username

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/152401
introduced a spec that relied on a hard-coded username (user1), but
this fails if another spec runs before it. Fix this spec by using the
actual username.

Relates to
https://gitlab.com/gitlab-org/quality/engineering-productivity/master-broken-incidents/-/issues/6249

ee/spec/helpers/sidebars_helper_spec.rb

@@ -228,7 +228,7 @@
       it 'returns paths for user' do
         expect(super_sidebar_context).to match(hash_including({
           sign_out_link: '/users/sign_out',
-          issues_dashboard_path: '/dashboard/issues?assignee_username=user1',
+          issues_dashboard_path: "/dashboard/issues?assignee_username=#{user.username}",
           merge_request_dashboard_path: nil,
           todos_dashboard_path: '/dashboard/todos',
           projects_path: '/dashboard/projects',