ci: Trigger build build on success

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

ci/Jenkinsfile

@@ -49,6 +49,10 @@ pipeline {
     }
 
     post {
+        success {
+            build "${BRANCH_NAME.replace('rust-cross/', 'build%2F')}"
+        }
+
         failure {
             sh 'unshare -Ur --map-auto chown root:root -R tmp log'
             dir('tmp/portage') {