Jenkinsfile: longer history, less artifacts

Keep more builds to get better statistics.
But throw away artifacts after 99 days (~a quarter year). 
This should clean outdated PRs a bit.

eclipse-platform/eclipse.platform.releng.aggregator#2653

Jenkinsfile

@@ -1,7 +1,7 @@
 pipeline {
 	options {
 		timeout(time: 40, unit: 'MINUTES')
-		buildDiscarder(logRotator(numToKeepStr:'15'))
+		buildDiscarder(logRotator(numToKeepStr: '100', artifactNumToKeepStr: '15', artifactDaysToKeep: '99'))
 		disableConcurrentBuilds(abortPrevious: true)
 		timestamps()
 	}