Fix image pull error due to missing sha256

[mbpavan]

Changes

The cronjob for tekton pruner fails as it cannot pull the image

failed to pull and unpack image "gcr.io/tekton-releases/dogfooding/tkn:8d4383ff675cdf

The image for example should have been "gcr.io/tekton-releases/dogfooding/tkn@sha256:8d4383ff675cdf (missing @sha256). This small change fixes it.

Fixes #2495

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[linux-foundation-easycla]

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• ✅ login: mbpavan (bf1f094)

[tekton-robot]

Hi @mbpavan. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mbpavan]

/easycla

[PuneetPunamiya]

/ok-to-test

[jkhelil]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jkhelil

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[jkhelil]

/lgtm