v0.37.x: Unscheduled Runs shouldn't be included in full status

[abayer]

Changes

Fixes #5282

When removing conditions, I accidentally made GetRunsStatus not skip including custom task ResolvedPipelineTasks without Runs in the PipelineRun.Status.Runs map. This resulted in cancellation failing, because the cancellation logic tries to patch each Run or TaskRun in the PipelineRun status (either in the Runs and TaskRuns maps for full embedded status, or ChildReferences for minimal or both embedded status). Unscheduled PipelineTasks for Tasks were already properly excluded from the status map, and ChildReferences was fine either way, but due to the oversight I mentioned above, unscheduled PipelineTasks for custom tasks were included in the full embedded status map, and cancellation would error due to that.

cherry-picked from #5287, with some additional modifications to child reference population for custom tasks without a Run which have been fixed in v0.38.x.

/kind bug

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Release Notes

Fix cancellation of PipelineRuns with unscheduled custom tasks.

[abayer]

/assign @afrittoli @pritidesai @dibyom @lbernick @jerop

[afrittoli]

/approve

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[lbernick]

/lgtm

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/reconciler/pipelinerun/pipelinerun.go	85.7%	86.3%	0.6
pkg/reconciler/pipelinerun/resources/pipelinerunstate.go	97.8%	97.8%	-0.0

[abayer]

/test check-pr-has-kind-label

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