Add support for specifiying "0" as no-timeout for PipelineRuns.

[dlorenc]

Changes

This was already done in #1040 for TaskRuns, but PipelineRuns seem to have been missed.
This fixes #1339

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Release Notes

Fix a bug with PipelineRuns timing out immediately when a timeout of 0 is specified.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dlorenc

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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/pipelinerun_types.go	95.5%	95.8%	0.4

[ghost]

/lgtm