ci: Trigger cross build on success

When the "base" image finishes building, Jenkins will start building a
new "cross" image on top of it.

ci/Jenkinsfile

@@ -55,6 +55,10 @@ pipeline {
     }
 
     post {
+        success {
+            build "${BRANCH_NAME.replace('base/', 'cross%2F')}"
+        }
+
         failure {
             sh 'unshare -Ur --map-auto chown root:root -R tmp log'
             dir('tmp/portage') {