The shared world-writable /tmp/pkh-keyrings directory, combined with the
skip-if-exists logic, let any local user pre-plant keyrings that pkh
then trusts into the chroot's trusted.gpg.d. Use a per-uid 0700
directory instead, refuse to reuse a pre-existing directory that is not
owned by the current user or is group/other-writable, and drop the now
unnecessary world-accessibility chmods (mmdebstrap in unshare mode runs
with the same real uid).