Inject the incus command, so the enumeration tests fail incus on every machine

The tests set MESH_LAB_INCUS in their body, which runs after the client module has read it, so they
asked the real incus: green where none is installed, red on the workstation that runs the lab.

test/enumerate.test.ts

@@ -1,11 +1,14 @@
-// Set before importing: the client reads MESH_LAB_INCUS once, at module load.
-// `false` is a real program that exits non-zero and prints nothing — which is also the worst
-// case, because an empty stderr is how a failure arrives with no explanation.
-process.env["MESH_LAB_INCUS"] = "false";
-
 import { test } from "node:test";
 import assert from "node:assert/strict";
-import { instanceExists, taggedInstances, taggedNetworks } from "../src/incus/client.ts";
+import { instanceExists, taggedInstances, taggedNetworks, useIncusCommand } from "../src/incus/client.ts";
+
+// `false` is a real program that exits non-zero and prints nothing — which is also the worst
+// case, because an empty stderr is how a failure arrives with no explanation.
+//
+// **Injected, never set in the environment here.** This file used to set MESH_LAB_INCUS in its body,
+// which runs after the imports above — so the client had already read the variable, and these tests
+// asked the real incus: green on a machine without one, red on the workstation that runs the lab.
+useIncusCommand(["false"]);
 
 /**
  * "I cannot see" must never be answered as "there is nothing there."