ReaR temporary build directory is included in backup when TMPDIR is defined in configuration

Issue

• When explicitly defining the TMPDIR environment variable in /etc/rear/{local,site}.conf, ReaR doesn't exclude the temporary build directory from the backup

  # grep -w TMPDIR /etc/rear/site.conf
  export TMPDIR=/temporary
  
  # rear mkbackup
  
  # tar tzvf /path/to/the/backup.tar.gz | grep "^temporary/rear"
  drwxr-xr-x root/root                0 2018-12-04 10:44 temporary/rear.XXX