Try to avoid a race where two rpmbuilds on the same node are trying to stage

Try to avoid a race where two rpmbuilds on the same node are trying to stage
the same rpm -- which ends up happening in the same directory. In order for
this race avoidance to work, the caller has to supply a unique TMPDIR which
rpmbuild will use at it's _tmpdir. Default to /var/tmp if not supplied.