Bug 25778: Fix var scope in koha-plack for PERL5LIB and KOHA_HOME

Bug 25778: Fix var scope in koha-plack for PERL5LIB and KOHA_HOME

Currently, if a koha-plack is given multiple instances, it will create
duplicate $KOHA_HOME/installer and $KOHA_HOME/installer/lib entries
in PERL5LIB as these changes are done at the global rather than local level.

This issue can only be seen in non-dev Koha, since dev Koha installs
rewrite PERL5LIB.

This patch localizes PERL5LIB to each instance.

To test dev installs:
0. cp debian/scripts/koha-plack /usr/sbin/koha-plack
1. koha-plack --restart kohadev
2. Note that nothing breaks

To test non-dev installs:
0. Don't replace /usr/sbin/koha-plack yet
1. Create multiple non-dev installs with plack enabled and running
2. Set up one of these non-dev installs so you can use the web
UI (that is beyong the scope of this test plan)
3. koha-plack --restart test1 test2 test3
4. For test3, go to /cgi-bin/koha/about.pl
5. Note that there are duplicate /usr/share/koha/installer
and /usr/share/koha/lib/installer entries in PERL5LIB
6. cp debian/scripts/koha-plack /usr/sbin/koha-plack
7. koha-plack --restart test1 test2 test3
8. For test3, go to /cgi-bin/koha/about.pl
9. Note that there are no duplicates entries in PERL5LIB

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>