config/prow: add k/utils to trigger_github_workflows trigger

[MadhavJivrajani]

Enable /retest trigger for GH workflows for k/utils.
Needed to unblock CI after kubernetes/utils#284
(can retrigger manually but not everyone can)

[ameukam]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ameukam, MadhavJivrajani

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[k8s-ci-robot]

@MadhavJivrajani: Updated the plugins configmap in namespace default at cluster test-infra-trusted using the following files:

• key plugins.yaml using file config/prow/plugins.yaml

In response to this:

Enable /retest trigger for GH workflows for k/utils.
Needed to unblock CI after kubernetes/utils#284
(can retrigger manually but not everyone can)

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