Try reusing name again.

limbo/limbo-trigger.yaml

@@ -38,7 +38,7 @@ spec:
   - apiVersion: tekton.dev/v1beta1
     kind: PipelineRun
     metadata:
-      generateName: $(params.repo_name)-pipeline-
+      generateName: "$(tt.params.repo_name)-pipeline-"
     spec:
       pipelineRef:
         name: limbo-pipeline