Jenkins CI: ROS msgs: replace GIT_BRANCH with BRANCH_NAME

Jenkinsfile

@@ -478,9 +478,10 @@ pipeline {
             docker { image 'px4io/px4-dev-base:2019-01-01' }
           }
           steps {
+            checkout(scm)
             sh('export')
             withCredentials([usernamePassword(credentialsId: 'px4buildbot_github_personal_token', passwordVariable: 'GIT_PASS', usernameVariable: 'GIT_USER')]) {
-              sh('git clone https://${GIT_USER}:${GIT_PASS}@github.com/PX4/px4_msgs.git -b ${GIT_BRANCH}')
+              sh("git clone https://${GIT_USER}:${GIT_PASS}@github.com/PX4/px4_msgs.git -b ${BRANCH_NAME}")
               sh('python msg/tools/uorb_to_ros_msgs.py msg/ px4_msgs/msg/')
               sh('cd px4_msgs; git status; git add .; git commit -a -m "Update message definitions `date`" || true')
               sh('cd px4_msgs; git push origin master || true')