ExitHandler not triggered on pipeline run cancellation

[rafalbigaj]

/kind bug

What steps did you take and what happened:
When cancelling a pipeline run, operations added with ExitHandler are not triggered.

Operations added with ExitHandler are mapped to the Tekton final tasks (finally).
Tekton final tasks are skipped when Tekton PipelineRun is cancelled.

What did you expect to happen:
A graceful pipeline run cancellation should guarantee that operations added with ExitHandler are triggered on run stop.

Additional information:
Related Argo issue: argoproj/argo-workflows#3155

Environment:

• Python Version (use python --version): 3.7
• SDK Version: 0.7.0
• Tekton Version (use tkn version): 0.22
• Kubernetes Version (use kubectl version): 1.18
• OS (e.g. from /etc/os-release):

[stale]

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