Add tarsnap/tarsnapper dir to PATH in cron job.

Often cron jobs are run with a minimal path.  For example, Ubuntu 12.04 cron
jobs have PATH=/usr/bin:/bin.  As a result, a cron entry with a relative path
(like the tarsnapper entry added by the tarsnap::tarsnapper recipe) will not
find an executable in /usr/local/bin.

Just making the cron entry use tarsnapper's absolute path is not sufficient
because tarsnapper later needs to find and execute tarsnap as well.