Bug#725434: workaround for injected TMPDIR

[David Schmitt]

This also affects sbuild. To avoid the issue, I've added the following to
my ~/.sbuildrc:

$external_commands = {
    "chroot-setup-commands" => [
        'runuser -u sbuild -- mkdir -v -m 1777 -p "/tmp/user/1000"',
    ],
};

Regards, David