Description of problem:

When a pipelinerun is defined with an on-cel annotation like body.pull_request.labels.exists(l, l == "run-the-test"), the pipelinerun does not run when a maintainer labels the PR with the "run-the-test" label. The on-cel expression is not currently evaluated at all when labels are updated unless the `on-label` annotation is present.

Workaround

Adding an on-label annotation to the pipelinerun allows the on-cel expression to be evaluated whenever a label is added. However the on-label annotation has no additional effect since the on-cel annotation means all other "on-*" annotations are ignored.

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

• Author a pipelinerun with an annotation like so: {{on-cel: event == "pull_request" && has(body.pull_request.labels) && body.pull_request.labels.exists(l, l.name == "run-the-test")
• Open a pull request with this pipelinerun
• Label the pull request with the "run-the-test" label

Actual results:

No pipelinerun is run

Expected results:

The pipelinerun starts

Reproducibility (Always/Intermittent/Only Once): alwaus

Acceptance criteria: 

• When an on-cel expression references a pull request's labels, changes to the PR labels can start the PLR if the on-cel expression matches

• (nice to have) in the above event, the PLR is only evaluated if the new label changes the evaluation of the on-cel expression result

Definition of Done:

Build Details:

Additional info (Such as Logs, Screenshots, etc):

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