fix(ci): preserve Debian apt package archives in linux-stage-zero

[nilbot]

Summary

In linux-stage-zero container jobs, Debian (debian:stable-slim) ships with /etc/apt/apt.conf.d/docker-clean, an APT hook that automatically deletes all downloaded .deb archives from /var/cache/apt/archives/ upon completion of apt-get commands. As a result, actions/cache was saving an empty 394-byte directory metadata tarball, preventing package archive caching.

Changes

• In .github/workflows/verify.yml, updated the Debian prereqs matrix entry to remove /etc/apt/apt.conf.d/docker-clean and set Binary::apt::APT::Keep-Downloaded-Packages "true"; in /etc/apt/apt.conf.d/keep-cache before running apt-get.
• Preserves downloaded Debian .deb packages in /var/cache/apt/archives/ so actions/cache can save and restore them across workflow runs.

Validation

• Validated YAML syntax in .github/workflows/verify.yml.
• Ran full Go test suites across agents and bootstrap.d: 100% PASS.
• Full two-stage subagent reviews (spec compliance and code quality): PASSED.