cups-filters: make shell string longer (close #10493)

The cstring for the shell path is too short for nixos in cups-filters,
causing it to be truncated.  This was previously fixed in #5428, but
regressed.

This is a permanent solution accepted even upstream
https://bugs.linuxfoundation.org/show_bug.cgi?id=1325
http://bzr.linuxfoundation.org/loggerhead/openprinting/cups-filters/revision/7401