fix(packages): restore generated tree ownership

[jmgilman]

Summary

• capture the mounted repository owner before running package-manager tools as root
• restore APT, RPM, and APK tree ownership after metadata generation
• keep host-side APT by-hash generation and APT/RPM signing writable on Linux runners

Production evidence

Package publication run 32545624161 passed all native package verification, then failed when the host process tried to create apt/dists/stable/main/binary-amd64/by-hash beneath a root-owned container output directory.

Verification

• go test ./internal/adapter/repogen