Bug 32856: Fix cataloguing/value_builder/unimarc_field_126.pl

Bug 32856: Fix cataloguing/value_builder/unimarc_field_126.pl

Use event parameter

This is part of the refactoring happening in bug 30975

Test plan:
1. Verify that the plugin continues to work as before

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 4fa7d59c61b524238b2e30f383ccda44e61e1096)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 57da913eae938a4968d4a9d3074ff807fef0b8cd)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>