tools/jenkins-jobs-builder: Ensure to use built-in Jenkins node

Errors can happen otherwise when executed on a different node.

This was missed when the job was turned to a pipeline one.

jobs/tools/jenkins-jobs-builder.groovy.j2

@@ -6,7 +6,9 @@ if (env.gitlabMergeRequestIid) {
 }
 
 pipeline {
-  agent any
+  agent {
+    label "built-in"
+  }
 
   stages {
 

jobs/tools/jenkins-jobs-builder.yaml

@@ -2,7 +2,6 @@
     name: jenkins-tools/{project_name}
     project-type: pipeline
     description: Update jenkins jobs and setup GitLab webhooks
-    node: built-in
     gitlab_project_name: swh/infra/ci-cd/swh-jenkins-jobs
     properties:
       - build-discarder: