_otter_env_core.sh: do not clobber existing PERL5LIB

Marek Szubarequested to merge
bugfix/env_perl5lib_exists into master

Created by: mkszuba

As otter_env_core.sh in both otter_live and otter_rel109 (as well as in the ensembl-otter repo; see the relevant PR there) stands, running otter overwrites whatever might have been present in $PERL5LIB. This is a problem e.g. if one uses this variable to make Perl aware of the install directory used by cpanminus. This PR ensures that the original value of PERL5LIB is preserved.

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