Do not trigger push pipeline if only tekton files are updated

Author: lmilbaum

.tekton/bootc-image-builder-push.yaml

@@ -7,8 +7,7 @@ metadata:
     build.appstudio.redhat.com/target_branch: "{{target_branch}}"
     pipelinesascode.tekton.dev/max-keep-runs: "3"
     pipelinesascode.tekton.dev/on-cel-expression:
-      event == "push" && target_branch
-      == "main"
+      event == "push" && target_branch == "main" && files.all.exists(x,!x.startsWith(".tekton/"))
   creationTimestamp: null
   labels:
     appstudio.openshift.io/application: bootc-image-builder