Fixes warnings about the non-existent `~/.nix-defexpr` folder when
using `use-xdg-base-directories = true`.
Example:
warning: Nix search path entry '/home/user/.nix-defexpr/channels' does not exist, ignoring
Starting Home Manager activation
Activating checkFilesChanged
Activating checkLinkTargets
Activating writeBoundary
Activating installPackages
replacing old 'home-manager-path'
installing 'home-manager-path'
Activating dconfSettings
Activating linkDesktopApplications
Activating linkGeneration
Cleaning up orphan links from /home/user
No change so reusing latest profile generation 13
Creating home file links in /home/user
Activating onFilesChange
Activating reloadSystemd
warning: Nix search path entry '/home/user/.nix-defexpr/channels' does not exist, ignoring
b0b0c3d943