ci: file scheduled-failure issues in rustfs/backlog (#7847)

Scheduled pipeline failures currently open [scheduled-failure] tracking
issues in rustfs/rustfs itself, adding bot noise to the code repository's
issue tracker. Route that alerting to rustfs/backlog instead.

- schedule-failure-issue gains target-repository (default rustfs/backlog);
  dedupe lookup, comment appends and issue creation target that repository.
- New source-token input (default ${{ github.token }}) reads the failed-job
  list from the reported run; the run still lives in and links to
  rustfs/rustfs. The issue body now carries a '- Repository:' line.
- All consumer workflows pass secrets.BACKLOG_ISSUE_TOKEN; the
  workflow-scoped GITHUB_TOKEN cannot open issues cross-repo. Alert job
  permissions blocks are unchanged.