grml2iso: make sure we have the sbin directories in our PATH

Under Debian/wheezy it fails out-of-the-box under sudo:

| sudo /usr/sbin/grml2iso -c ./source/templates -o grml64-sipwise-release_20130626.iso grml64-full_2013.02.iso
| Error: Could not find grml2usb executable. Is /usr/sbin missing in PATH?

because of:

| ENV_SUPATH      PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
| ENV_PATH        PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

in /etc/login.defs.