lab: take the forge's owner as a required config value, and name no node in its fixtures (issue 1838)

The lab cloned and installed from the owner this mesh's repositories are under, written in its code; another
mesh's forge holds them under its own (1838/AC4). Unset, lab_check, lab_run and a rebuild say so. Fixtures
name neutral machines and an example owner.

modules/lab/files/lab.env

@@ -1,3 +1,4 @@
 MESH_LAB_FORGE=${config:forge}
+MESH_LAB_FORGE_OWNER=${config:forge-owner}
 MESH_LAB_FORGE_BINDING=${dir:state}/package-registry.json
 MESH_LAB_FORGE_SECRET=${dir:state}/package-registry.secret