The use-xdg-base-directories Nix setting can be set globally in
/etc/nix/nix.conf, in which case Home Manager doesn't know about it.
Users could fix that by also setting it, redundantly, in `nix.settings`,
but then Nix issues a lot of spurious warnings about
use-xdg-base-directories being a restricted setting that untrusted users
can't pass on to the daemon.
As an alternative, users can now set `nix.assumeXdg`, which makes Home
Manager assume that use-xdg-base-directories is in effect without adding
it to the user's nix.conf file.
