Use --tmpdir with mktemp

Pass --tmpdir to mktemp so the new temporary directory is placed
within $TMPDIR, or falling back to /tmp. This way dib-run-parts can
use whatever the temporary directory is, instead of using an hardcoded
/tmp.

Change-Id: I6b55e3c6c1fe22108ea555d3766e0323695bd96d
Merge "Use --tmpdir with mktemp"