Convert times to UTC

[zregvart]

Changes

The SLSA provenance prescribes that the Timestamp data types must be in UTC time zone. It can happen that the environment where Chains controller is running is not set to the UTC time zone, and in that case the predicate.metadata.buildStartedOn and
predicate.metadata.buildFinishedOn will be set with the local time zone. This is due to apimachinery converting time.Time values into local timezone on JSON unmarshall.

This forces all dates to the UTC time zone.

Fixes #630

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As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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Release Notes

Makes sure all dates in SLSA provenance attestations are in UTC time zone

[tekton-robot]

Hi @zregvart. 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[lcarva]

/ok-to-test

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/chains/formats/intotoite6/pipelinerun/pipelinerun.go	77.3%	77.8%	0.5
pkg/chains/formats/intotoite6/taskrun/taskrun.go	76.5%	77.1%	0.7

[wlynch]

/lgtm

Thanks!

[tekton-robot]

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This pull-request has been approved by: wlynch

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