Inhibit kured on CI cluster upgrade

Looks like kured is triggering node reboots in the middle of our
testing which results in a very unstable job.

This patch reuses the same inhibit_kured step that we used on
other jobs to prevent kured from restarting nodes while we manipulate
them

ci/jenkins/pipelines/skuba-upgrade-from-4.2.Jenkinsfile

@@ -65,7 +65,11 @@ pipeline {
                 }
             }
         }
-
+
+        stage('Inhibit kured reboots') { steps {
+            sh(script: 'make -f ci/Makefile inhibit_kured')
+            }
+        }
         stage('upgrade skuba') { 
             steps {
                 sh(script: "sudo zypper in -y --no-recommends go1.13")