Validate Context Variables in Finally Tasks

[jerop]

Changes

Prior to this change, Context Variables were not validated in
Finally Tasks. This change adds that validation and its tests.

/kind bug

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Release Notes

Added validation for Context variables in Finally Tasks

[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/v1beta1/pipeline_validation.go	99.6%	99.6%	0.0

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbwsg

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[pritidesai]

thanks @jerop
/lgtm

[jerop]

/test check-pr-has-kind-label