Set "startedAt" in skipped case

[vincent-pli]

Fix issue: #2416

The root cause is in skipped case, entrypoint do not write startedAt to Termination file.
That's means when reconcile of taskrun try to update startedAt, they found nothing, and the result is the startedAt will keep as the startedAt in pod.

Changes

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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/entrypoint/entrypointer.go	55.3%	56.1%	0.8

[vdemeester]

/cc @imjasonh @bobcatfish

[vdemeester]

/meow

[tekton-robot]

@vdemeester:

In response to this:

/meow

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[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[ghost]

@vincent-pli Could you update your commit message to match the PR message? It makes browsing our git history much more helpful in the future.

[vincent-pli]

@sbwsg
Commit message is update, thanks

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/entrypoint/entrypointer.go	55.3%	56.4%	1.1

[vincent-pli]

/test pull-tekton-pipeline-integration-tests

[ghost]

/lgtm