When using the NixOS module we cannot guarantee that the Nix store
will be writable during startup. Installing the user packages through
`nix-env -i` will fail in these cases.
This commit adds a NixOS option `home-manager.useUserPackages` that,
when enabled, installs user packages through the NixOS
users.users.<name?>.packages
option.
Note, when submodule support and external package install is enabled
then the installed packages are not available in `~/.nix-profile`. We
therefore set `home.profileDirectory` directly to the HM profile
packages.
ef168979bf