nanobsd: guard triggers cleanup find against missing dir

[senguptaangshuman17_gmail.com]

Summary

find(1) errors on a nonexistent path, and under set -e that
kills the build. var/db/pkg/triggers only exists if pkg(8)
actually created it during install, so check -d before find
in _xxx_tgt_pkg_triggers instead of assuming it's there.

[jlduran]

This revision should be squashed with D59041.