chore: re-enable harden-runner in nightly build

[kaushalnavneet]

Changes

• Re-enabled security hardening step in nightly build workflow, now that gsutil is no longer used (switched to OCI buckets).

Fixes #4975

/kind misc

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[AlanGreene]

Thanks. I'll kick off a run manually to verify once this is merged 🤞
/lgtm

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