reproducible Debian: mark cbxi4a as offline (needs investigation why)

Signed-off-by: Holger Levsen <holger@layer-acht.org>

jenkins-home/offline_nodes

@@ -17,5 +17,8 @@
 # ethernet not recognized
 odxu4b-armhf-rb.debian.net
 
+# needs investigation
+cbxi4a-armhf-rb.debian.net
+
 # Down here nodes are automatically added by the maintenance job when they have
 # been failing their health check for too long.