[ci] Fix MissingPropertyException - COTS-UE by binding OAI_Registry globally

`def` creates a variable local to the script object, not automatically visible inside helper functions.

This fixes the following exception in Jenkins:

    Found unhandled groovy.lang.MissingPropertyException exception:
    No such property: OAI_Registry for class: groovy.lang.Binding

ci-scripts/Jenkinsfile-GitLab-Container

@@ -35,7 +35,7 @@ def gitCommitAuthorEmailAddr
 // list of failing stages
 def failingStages = ""
 
-def OAI_Registry = "gracehopper3-oai.sboai.cs.eurecom.fr"
+OAI_Registry = "gracehopper3-oai.sboai.cs.eurecom.fr"
 
 pipeline {
   agent {