Sort matrix TaskRuns by display name on the PipelineRun details page

[AlanGreene]

Changes

Related to #3613

To ensure a stable order when showing the matrix TaskRuns in the UI, sort by display name. This should also work for custom tasks that produce multiple TaskRuns.

If there is no display name we maintain the source order, so there is no change to the user experience in that case.
/kind feature

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Release Notes

Use TaskRun display names to provide a stable sort order when displaying matrixed TaskRuns on the PipelineRun details page.

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[briangleeson]

/lgtm

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