Allow overriding extension binary to local path

[stbenjam]

This makes it easier to do local development.

[smg247]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: smg247, stbenjam

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[openshift-ci]

@stbenjam: The following tests 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/e2e-vsphere-ovn-upi	8285ecf	link	true	/test e2e-vsphere-ovn-upi
ci/prow/e2e-metal-ipi-ovn	8285ecf	link	false	/test e2e-metal-ipi-ovn
ci/prow/e2e-metal-ipi-ovn-kube-apiserver-rollout	8285ecf	link	false	/test e2e-metal-ipi-ovn-kube-apiserver-rollout
ci/prow/e2e-hypershift-conformance	8285ecf	link	false	/test e2e-hypershift-conformance
ci/prow/e2e-aws-ovn-single-node-upgrade	8285ecf	link	false	/test e2e-aws-ovn-single-node-upgrade
ci/prow/e2e-openstack-ovn	8285ecf	link	false	/test e2e-openstack-ovn
ci/prow/okd-scos-e2e-aws-ovn	8285ecf	link	false	/test okd-scos-e2e-aws-ovn
ci/prow/e2e-aws-ovn-kube-apiserver-rollout	8285ecf	link	false	/test e2e-aws-ovn-kube-apiserver-rollout

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