Set PERL_CARTON_EXEC=1 variable upon exec

Scenario: You have a legacy app that is now carton-ized. You have a bunch of scripts that don't use carton, and you can't expect people in your teamto explicitly call "carton exec -- script.pl". So you want to re-execute these scripts via carton and set the right paths when somebody casually does "./bin/foo.pl".

This change allows sets $ENV{PERL_CARTON_EXEC} variable to true, so called scripts can detect if it has been dispatched via carton or not.