nixos/build-vms: propagate file location

When trying to build a VM using `nixos-build-vms` with a configuration
that doesn't evaluate, an error "at `<unknown-file>`" is usually shown.

This happens since the `build-vms.nix` creates a VM-network of
NixOS-configurations that are attr-sets or functions and don't contain
any file information. This patch manually adds the `_file`-attribute to
tell the module-system which file contained broken configuration:

```
$ cat vm.nix
{ vm.invalid-option = 1; }

$ nixos-build-vms vm.nix
error: The option `invalid-option' defined in `/home/ma27/Projects/nixpkgs/vm.nix@node-vm' does not exist.
(use '--show-trace' to show detailed location information)
```

nixos/modules/installer/tools/nixos-build-vms/build-vms.nix

@@ -3,7 +3,12 @@
 , networkExpr
 }:
 
-let nodes = import networkExpr; in
+let
+  nodes = builtins.mapAttrs (vm: module: {
+    _file = "${networkExpr}@node-${vm}";
+    imports = [ module ];
+  }) (import networkExpr);
+in
 
 with import ../../../../lib/testing-python.nix {
   inherit system;