Remove interactive ISO storage detection/selection and associated ISO_PATH logic. Default to a fixed cache directory (/var/lib/vz/template/iso) via CACHE_DIR and update CACHE_FILE and mkdir usage accordingly. Also update VM configuration to reference the local ISO store (local:iso/<file>) instead of the previously selectable storage. This simplifies ISO handling by avoiding pvesm/whiptail prompts; note this hardcodes the ISO location and removes support for alternate ISO storages.
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