Bug 28367: Fix plack condition in C4/Auth_with_shibboleth.pm

Bug 28367: Fix plack condition in C4/Auth_with_shibboleth.pm

C4/Auth_with_shibboleth.pm has:
  if ( any { /(^psgi\.|^plack\.)/i } keys %ENV )
When others have:
  if ( any { /(^psgi\.|^plack\.)/i } keys %ENV )

This has been highlighted by https://gitlab.com/koha-community/koha-testing-docker/-/issues/249 that introduces a PLACK_WORKERS env vars, and t/Auth_with_shibboleth.t was failing with

 #   Failed test 'checkpw_shib tests'
 #   at t/Auth_with_shibboleth.t line 335.
 Invalid parameter passed, categorycode= does not exist# Looks like your test exited with 255 just after 5.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>