Motivation for this change

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)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.