use knative-extensions org for istio install

[dprotaso]

Fixes: #6504

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[dprotaso]

/cherry-pick release-1.20

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.20 in a new PR and assign it to you.

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In response to this:

/cherry-pick release-1.20

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[Cali0707]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, dprotaso

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• OWNERS [Cali0707,dprotaso]

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[knative-prow-robot]

@dprotaso: new pull request created: #6511

Details

In response to this:

/cherry-pick release-1.20

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-sigs/prow repository.