Fix race condition on LFS Token

## What does this MR do?

 Fixes a race condition that can be triggered if the token expires right after we retrieve it, but before we can set the new expiry time.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6551#note_16190569

Thanks for catching this so quickly @jacobvosmaer-gitlab

cc @DouweM

See merge request !6592
Signed-off-by: Rémy Coutable <remy@rymai.me>