support PERLBREW_ROOT environment variable

Hi, we have several users that share a perlbrew installation, so it may live at /opt/perlbrew for instance and we indicate this to perlbrew with PERLBREW_ROOT. This change fixes an error I've run into installing Ubic:

t/setup.t ...................... Can't find perlbrew config (assumed /root/perl5/perlbrew/etc/bashrc) at /home/a1155544/local/src/Ubic/.build/JO9Ady32pG/lib/Ubic/Admin/Setup.pm line 295.

Cheers