Give the integration level the node's installed Node toolchains read-only, so the Node build module's tests build offline (issue 1891, ADR 0533)

mesh-build-go binds /var/lib/mesh-build-node-cache/toolchain read-only into the
integration level's one shut-off process when it is there; the unit level never.
mesh-build-node opens its installed toolchains to other accounts (folders 0755,
files 0644, writable by its owner alone; its cache 0711, so the npm cache beside
them stays its own), and realMachine copies the toolchain of its own lock from
there instead of npm ci from the registry, which the level cannot reach.

modules/mesh-build-node/module.json

@@ -87,7 +87,7 @@
       "id": "cache",
       "type": "directory",
       "path": "/var/lib/mesh-build-node-cache",
-      "mode": "0700",
+      "mode": "0711",
       "owner": "mesh-build-node",
       "unknown": "program"
     },