Enable RequestStore for pipelines controller N+1 spec
I pushed this to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1849 to fix https://gitlab.com/gitlab-org/gitlab-ee/builds/15880645, but it should be in CE too to avoid conflicts (and we already use this pattern in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/serializers/pipeline_serializer_spec.rb).