mkisoimages.sh: Avoid creating temporary files in the current directory

Currently the current directory is the parent of the rootfs directory,
but this will change in order to support NO_ROOT builds that use a
metalog manifest, since those need to have the current directory be the
rootfs itself in order for the relative paths to be correct, and we do
not want the non-METALOG case (which passes the directory to makefs) to
pick up leftover temporary .img files from a previous failed build.

Reviewed by: brooks, emaste, gjb
Differential Revision: https://reviews.freebsd.org/D33998