YT-381: Use gitbot token for custom projects

  Build process uses GITLAB_PRIVATE_TOKEN for multiple upstream repositories,
  including private SECO repos.
  However, the new token rotation mechanism does not support variables
  with multiple tokens, making this approach inconvenient.

  This changes:
   - Use GITBOT_TOKEN for SECO repositories.
   - Keep GITLAB_PRIVATE_TOKEN for other upstream repos.