Make buildConfigSource test support non-deterministic ref

[joejstuart]

This test checks to make sure the ref value returned is contained in the Digest.

Changes

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Release Notes

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[tekton-robot]

Hi @joejstuart. Thanks for your PR.

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[lcarva]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcarva

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[lcarva]

/ok-to-test