let the operator set, replace and remove an agent's backend env from the swarm ui

nix/host-modules/swarm-bao.nix

@@ -474,6 +474,10 @@ let
       capabilities = ["delete"]
     }
 
+    path "${credentialMountPath}/metadata/swarm/agents/+/backend-env" {
+      capabilities = ["delete"]
+    }
+
     path "${credentialMountPath}/data/swarm/hives/+/matrix/sender-token" {
       capabilities = ["create", "read", "update"]
     }