ci: Do not wait for downstream job

Rather, finish immediately and do not account for the downstream job's
status in our own.

ci/Jenkinsfile

@@ -50,7 +50,10 @@ pipeline {
 
     post {
         success {
-            build "${BRANCH_NAME.replace('rust-cross/', 'build%2F')}"
+            build([
+                job: "${BRANCH_NAME.replace('rust-cross/', 'build%2F')}",
+                wait: false,
+            ])
         }
 
         failure {