Bug 36517: Fix output from install_plugins.pl

Bug 36517: Fix output from install_plugins.pl

When misc/devel/install_plugins.pl does not find any plugins, it prints
the list of pluginsdir, but with a literal \n separating the dirs, and
no newline at the end.

To test:
- Edit koha-conf.xml and add a second entry for <pluginsdir>, so there
  are two entries. The second one could just be a copy of the original.
- Run "perl misc/devel/install_plugins.pl"
- Note the output looks something like this:

No plugins found
pluginsdir contains:
/var/lib/koha/kohadev/plugins\n/var/lib/koha/kohadev/pluginsroot@kohadevbox:koha(master)$

- Apply the patch and run the script again. Output should be:

No plugins found
pluginsdir contains:
/var/lib/koha/kohadev/plugins
/var/lib/koha/kohadev/plugins
root@kohadevbox:koha(master)$

- Sign off

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>