Pass CACHE_UPDATE variable to omnibus-gitlab-mirror if present

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

scripts/trigger-build

@@ -153,7 +153,8 @@ module Trigger
         'ALTERNATIVE_SOURCES' => 'true',
         'SECURITY_SOURCES' => Trigger.security? ? 'true' : 'false',
         'ee' => Trigger.ee? ? 'true' : 'false',
-        'QA_BRANCH' => ENV['QA_BRANCH'] || 'master'
+        'QA_BRANCH' => ENV['QA_BRANCH'] || 'master',
+        'CACHE_UPDATE' => ENV['OMNIBUS_GITLAB_CACHE_UPDATE']
       }
     end
   end