MT#9683 Overwrite TMPDIR in pre_install_add_sipwise_repos.sh

Otherwise fails with:

| 0m8.8s ERROR: Command failed (status=1): ['chroot', '/var/cache/pbuilder/tmp.upxyAfmDXA_piuparts_tmp/tmpN0qCK0', 'tmp/scripts/pre_install_add_sipwise_repos.sh']
|   ** Setting up http://deb.sipwise.com/autobuild/680FBA8A.asc for apt-get usage ***
|   mktemp: failed to create file via template `/var/cache/pbuilder/tmp.upxyAfmDXA_piuparts_tmp/tmp.XXXXXXXXXX': No such file or directory

scripts/pre_install_add_sipwise_repos.sh

@@ -17,6 +17,8 @@ if [ -z "$release" ] ; then
   echo "*** No release variable set, using default [$release] ***"
 fi
 
+TMPDIR='/tmp' # make sure we don't get the $TMPDIR pointing to piuparts' tmpfs
+
 # make sure we can rely on wget being present for checks
 type wget >/dev/null 2>&1 || apt-get --allow-unauthenticated -y install wget