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.
This removes the `nixosSubmodule` option in favor of a new option
`submoduleSupport.enable`. This name better indicates that the
submodule mode applies to both NixOS and nix-darwin.
This is a NixOS module that is intended to be imported into a NixOS
system configuration. It allows the system users to be set up directly
from the system configuration.
The actual profile switch is performed by a oneshot systemd unit per
configured user that acts much like the regular `home-manager switch`
command.
With this implementation, the NixOS module does not work properly with
the `nixos-rebuild build-vm` command. This can be solved by using the
`users.users.<name?>.packages` option to install packages but this
does not work flawlessly with certain Nixpkgs packages. In particular,
for programs using the Qt libraries.
