script: ignore main branch when cleaning ci

script: ignore main branch when cleaning ci

Addresses failures like:

     Deleting refs: refs/heads/main refs/heads/wip-athakkar-testing-2025-02-11-1344 refs/heads/wip-choffman-fscrypt-021225 refs/heads/wip-dang-68336 refs/heads/wip-leonidc-7_1_hotfix refs/heads/wip-nia-testing-2025-02-12-0101 refs/heads/wip-nitzan-gil-perf-count-and-contention-detect refs/heads/wip-rf-perf-sections refs/heads/wip-sseshasa-test-stuck-backfill-squid-feb11-2024 refs/heads/wip-sseshasa-test-stuck-backfill-squid-feb12-2024 refs/heads/wip-yuri8-testing-2025-02-10-2350
    To https://github.com/ceph/ceph-ci
     - [deleted]           wip-athakkar-testing-2025-02-11-1344
     - [deleted]           wip-choffman-fscrypt-021225
     - [deleted]           wip-dang-68336
     - [deleted]           wip-leonidc-7_1_hotfix
     - [deleted]           wip-nia-testing-2025-02-12-0101
     - [deleted]           wip-nitzan-gil-perf-count-and-contention-detect
     - [deleted]           wip-rf-perf-sections
     - [deleted]           wip-sseshasa-test-stuck-backfill-squid-feb11-2024
     - [deleted]           wip-sseshasa-test-stuck-backfill-squid-feb12-2024
     - [deleted]           wip-yuri8-testing-2025-02-10-2350
     ! [remote rejected]   main (refusing to delete the current branch: refs/heads/main)
    error: failed to push some refs to 'https://github.com/ceph/ceph-ci'
    Error: Process completed with exit code 1.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>