locate.updatedb: Revert to using cat to copy the db.

This script is usually run unprivileged, so install fails to create a
temporary file while copying the finished database. Revert to using
cat, which can overwrite the existing file as it is usually owned by
the same user which is running the script.

Fixes: f62c1f3f8e91
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D46872