vmimage.subr: Fix NO_ROOT build in ..._rmcache

When doing a NO_ROOT build we can't run pkg inside a chroot, for
multiple reasons including not having /dev mounted.

MFC after: 6 hours
Sponsored by: https://www.patreon.com/cperciva

(cherry picked from commit 86221f965c06efa1cd5b2da640ae23edded7c8b9)