Add validation for array indexing in finally when expressions

[lbernick]

Currently, the pipelines reconciler validates that array indexing references to parameters are not out of bounds. Prior to this commit, validation was accidentally skipped for pipeline.spec.finally.when.inputs. This commit adds this validation.

/kind bug

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Release Notes

Bug fix: add validation for out-of-bounds indexing into array parameters referenced in pipeline.spec.finally.when.inputs

[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/apis/pipeline/v1/pipeline_validation.go	99.4%	99.4%	0.0
pkg/apis/pipeline/v1beta1/pipeline_validation.go	99.1%	99.1%	0.0

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1/pipeline_validation.go	99.4%	99.4%	0.0
pkg/apis/pipeline/v1beta1/pipeline_validation.go	99.1%	99.1%	0.0

[Yongxuanzhang]

Thanks!
This is the function where we replace the when expressions, and array indexing is also applied to when's input but missed in validation.

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jerop, Yongxuanzhang

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