nixos/lib/make-disk-image.nix: fix systemd-boot-builder clobbering /homeless-shelter

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).

nixos/tests/qemu-vm-external-disk-image.nix

@@ -69,5 +69,8 @@ in
     os.environ['NIX_DISK_IMAGE'] = tmp_disk_image.name
 
     machine.succeed("findmnt --kernel --source ${rootFsDevice} --target /")
+
+    # Make sure systemd boot didn't clobber this
+    machine.succeed("[ ! -e /homeless-shelter ]")
   '';
 }