Updating .ci-operator.yaml build_root_image from openshift/release

This is an autogenerated PR that updates the `.ci-operator.yaml`
to reference the `build_root_image` found in the [ci-operator-config](https://github.com/openshift/release/tree/master/ci-operator/config)
in the [openshift/release](https://github.com/openshift/release) repository.

This is done in preparation for enabling reading the `build_root` from
your repository rather than the central config in [openshift/release](https://github.com/openshift/release).
This allows to update the `build_root` in lockstep with code changes. For details, please
refer to the [docs](https://docs.ci.openshift.org/docs/architecture/ci-operator/#build-root-image).

Note that enabling this feature is mandatory for all OCP components that have an ART build config.

A second autogenerated PR to the [openshift/release repository](https://github.com/openshift/release)
will enable reading the `build_root` from your repository once this PR was merged.

If you have any questions, please feel free to reach out in the #forum-testplatform
channel in the CoreOS Slack.

Author: openshift-ci-robot

.ci-operator.yaml

@@ -0,0 +1,4 @@
+build_root_image:
+  name: release
+  namespace: openshift
+  tag: rhel-7-release-openshift-4.6