Flush cache and always (re)generate jobs

This means that we always regenerate all jobs, overwriting any
changes which may have been (manually) made.

Change-Id: I1d8f91117ed903ad7af842ed8d0753866a46f859
diff --git a/jobs.yaml b/jobs.yaml
index 88c6b97..1d9d765 100644
--- a/jobs.yaml
+++ b/jobs.yaml
@@ -94,7 +94,7 @@
                   branch-pattern: 'master'
           silent: true
     builders:
-      - shell: "jenkins-jobs --conf /var/lib/jenkins/jenkins_jobs.conf update jobs.yaml"
+      - shell: "jenkins-jobs --flush-cache --conf /var/lib/jenkins/jenkins_jobs.conf update jobs.yaml"
 
 - job:
     name: update-jenkins-verify