ci: post: Trigger correct job, do not wait

The next job in the pipeline is _rust-cross/main_, not
_cross-rust/main_.

Also, we don't need to wait for follow-up jobs.  Otherwise, we end up
with the whole pipeline blocking the _base_ job, etc.

ci/Jenkinsfile

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