cleanup(USB PR 2): retire legacy installer substrate — delete infra/nixos/hosts/installer/ + build-installer-iso.yml + update root flake; add B-0830 follow-up#5320

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otto-cli/usb-cleanup-pr2-investigation-infra-installer-workflow-consolidation-2026-05-26Lucent-Financial-Group/Zeta:otto-cli/usb-cleanup-pr2-investigation-infra-installer-workflow-consolidation-2026-05-26Copy head branch name to clipboard