Add Circle CI API token to request header if available (#26519)

Follow up of #26499
A Circle CI team member got back to me. It is indeed not necessary, but
they had a regression not long ago on fetching without token.

https://discuss.circleci.com/t/is-api-token-required-when-fetching-artifacts/47606/5

To mitigate the impact of this kind of issues, let's add this token to
requests' header when it's available.