Set up prow job to automatically build and push JobSet image to GCR

[danielvegamyhre]

See kubernetes-sigs/jobset#58

cc @ahg-g

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danielvegamyhre, dims

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[k8s-ci-robot]

@danielvegamyhre: Updated the job-config configmap in namespace default at cluster test-infra-trusted using the following files:

• key k8s-staging-jobset.yaml using file config/jobs/image-pushing/k8s-staging-jobset.yaml

In response to this:

See kubernetes-sigs/jobset#58

cc @ahg-g

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