Allow setting a channel for new users to auto-join (#9291)

Release Notes:

- Automatically add new users to the #zed channel

crates/collab/k8s/collab.template.yml

@@ -185,6 +185,8 @@ spec:
               value: "true"
             - name: ZED_ENVIRONMENT
               value: ${ZED_ENVIRONMENT}
+            - name: AUTO_JOIN_CHANNEL_ID
+              value: ${AUTO_JOIN_CHANNEL_ID}
           securityContext:
             capabilities:
               # FIXME - Switch to the more restrictive `PERFMON` capability.