Use FQDN for ghproxy endpoint in prow jobs

[oliver-goetz]

/kind bug

What this PR does / why we need it:
Use FQDN for ghproxy endpoint in prow jobs because prow job pods run in test-pods namespace.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

[gardener-prow]

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[gardener-prow]

LGTM label has been added.

Git tree hash: 6a14ceb7af0bccbb77ef387ed98e7b7712a5e249

[gardener-prow]

@oliver-goetz: Updated the job-config configmap in namespace prow at cluster gardener-prow-trusted using the following files:

• key ci-infra-periodics.yaml using file config/jobs/ci-infra/ci-infra-periodics.yaml

In response to this:

/kind bug

What this PR does / why we need it:
Use FQDN for ghproxy endpoint in prow jobs because prow job pods run in test-pods namespace.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.