change master branch detection (#24383)

src/eo/Utils.groovy

@@ -9,7 +9,7 @@ def mail_notify(currentBuild, env, email) {
         // Hack to have the 'jenkins build back to normal' mail sent
         currentBuild.result = 'SUCCESS'
     }
-    if (env.BRANCH_NAME == 'master') {
+    if (env.GIT_BRANCH == 'origin/master') {
         step([
             $class: 'Mailer', notifyEveryUnstableBuild: true,
             recipients: email, sendToIndividuals: true