Fix no repeated subjects for v2alpha4 pipelineruns

[renzodavid9]

Fixes: #1131

Change the pipelinerun v2alpha4 method that calculates the subjects to use artifact.AppendSubjects instead of append to avoid duplicated subjects.

Changes

Use of artifact.AppendSubjects instead of append when calculating subjects with artifacts.pipelinerun.enable-deep-inspection: "true" for a pipelinerun using v2alpha4 formatter.

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[tekton-robot]

Hi @renzodavid9. Thanks for your PR.

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[chitrangpatel]

/ok-to-test

[chitrangpatel]

Thanks for the quick fix @renzodavid9 !! /approve /lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel

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[renzodavid9]

Rebased with latest changes fixing t _type and predicateTpe fields, those should fix failing integration tests here

[chitrangpatel]

/lgtm

[chitrangpatel]

/kind bug