management cluster: Add missing rancher-agent airgap image

It has been reported that the rancher installation has failures
when this image is not included, evidently it's not derived from
the EIB inspection of the chart.

Author: hardys

asciidoc/product/atip-management-cluster.adoc

@@ -1162,6 +1162,7 @@ embeddedArtifactRegistry:
     - name: registry.rancher.com/rancher/rancher-webhook:v0.8.1
     - name: registry.rancher.com/rancher/rancher/turtles:v0.24.0
     - name: registry.rancher.com/rancher/rancher:v2.12.1
+    - name: registry.rancher.com/rancher/rancher-agent:v2.12.1
     - name: registry.rancher.com/rancher/shell:v0.4.1
     - name: registry.rancher.com/rancher/system-upgrade-controller:v0.16.0
     - name: registry.suse.com/rancher/cluster-api-controller:v1.10.5