fix: add nightly registry to registryOverride

[ahitacat]

https://redhat.atlassian.net/browse/ARO-25997

What

This PR adds the nightly registry to the overrideRegistry, so we can use nightly ocp release images for cluster creation.

Why

Fix the bug described in https://redhat.atlassian.net/browse/ARO-25997

Testing

Manual test pending

Special notes for your reviewer

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ahitacat
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[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[openshift-ci]

@ahitacat: 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/cspr	ebf1b5e	link	true	/test cspr

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