.cci.jenkinsfile: add build discarder to rotate logs

Limit build history to the 10 most recent runs or 30 days. This keeps
PVC storage space down when there are long-running PRs in this repo with
frequent force-pushes.

Written by the Pipeline-Syntax Snippet Generator in the Jenkins UI.

Author: marmijo

.cci.jenkinsfile

@@ -2,7 +2,14 @@
 
 properties([
     // abort previous runs when a PR is updated to save resources
-    disableConcurrentBuilds(abortPrevious: true)
+    disableConcurrentBuilds(abortPrevious: true),
+
+    // only keep the last 10 builds
+    // builds older than 30 days are also rotated out
+    buildDiscarder(logRotator(
+        numToKeepStr: '10',
+        daysToKeepStr: '30'
+    ))
 ])
 
 // Grab a decent amount of memory/cpu