Jenkinsfile: run pipelineOpenSTA on k8s pods

Switch to the k8s-pipeline-opensta branch of the utils shared library
and wrap pipelineOpenSTA in a coordinator k8sPodTemplate so the
top-level dispatch runs on a Kubernetes pod, mirroring OpenROAD's
Jenkinsfile.

Do not merge until the corresponding jenkins-ci PR
(The-OpenROAD-Project-private/jenkins-ci#136) is merged and tagged --
this will need a follow-up to pin @utils to the released tag.

Jenkinsfile

@@ -1,4 +1,5 @@
-@Library('utils@main') _
-node() {
-    pipelineOpenSTA()
+@Library('utils@k8s-pipeline-opensta') _
+
+k8sPodTemplate(coordinator: true, cloud: utilPickCloud()) {
+    pipelineOpenSTA(k8s: true)
 }