allow go previous is none

[Ximinhan]

GO_PREVIOUS will be removed so only have latest

[openshift-ci]

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[joepvd]

We may need this in the future. Possibly better to query if this var is set, and act if so?

[openshift-ci]

@Ximinhan: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	a7c5174	link	false	/test security

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[thegreyd]

/lgtm