systemd-boot-builder.py calls nix-env --list-generations which creates
$HOME/.nix-defexpr/channels/nixos if it doesn't exist. This would cause a folder
/homeless-shelter to show up in the final image which in turn breaks nix builds
in the target image if sandboxing is turned off (as /homeless-shelter is never
allowed to exist).
Allow the user to disable overriding the fileSystems option with
virtualisation.fileSystems by setting
`virtualisation.fileSystems = lib.mkForce { };`.
With this change you can use the qemu-vm module to boot from an external
image that was not produced by the qemu-vm module itself. The user can
now re-use the modularly set fileSystems option instead of having to
reproduce it in virtualisation.fileSystems.
