draft: schedule to run monthly

ci/Jenkinsfile

@@ -1,5 +1,9 @@
 // vim: set ft=groovy sw=4 ts=4 sts=4 et :
 
+def IS_TRIGGERED_BY_TIMER = currentBuild.getBuildCauses(
+    'hudson.triggers.TimerTrigger$TimerTriggerCause'
+).size() > 0
+
 pipeline {
     agent {
         kubernetes {
@@ -16,9 +20,14 @@ pipeline {
         disableConcurrentBuilds()
     }
 
+    triggers {
+        cron 'TZ=America/Chicago\n20 10 31 10 *'
+    }
+
     parameters {
         booleanParam \
             name: 'Clean',
+            defaultValue: IS_TRIGGERED_BY_TIMER,
             description: 'Clean the workspace and perform a full rebuild'
     }