fix(build): drop cached archzfs packages before mkarchiso runs

The upstream archzfs mirror has handed us corrupted .pkg.tar.zst files several times now. Past builds aborted at pacstrap with "File X is corrupted (invalid or corrupted package (checksum))". The cached package sits in /var/cache/pacman/pkg/ between builds, so once it's bad every subsequent build fails until someone manually deletes it.

I added a step right before mkarchiso that rm -f's the archzfs entries (zfs-dkms-*, zfs-utils-*) from the host pacman cache. Pacstrap fetches fresh copies on the next build. Costs ~30s of bandwidth on a warm mirror. Saves debugging a baffling pacstrap abort.

The proper permanent fix is a local caching proxy (pacoloco or flexo). That lives on the Medium Lifts backlog. This commit is the cheap interim that addresses the recurring failure mode without new infrastructure.

Shellcheck reports clean. The rm -f globs handle empty-match cleanly (silent return). I skipped a full build re-run since the parent commit (b4c0f72) already verified the surrounding mkarchiso path. This is a defensive prepend with no interaction surface.

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