This changes the installation command from
nix-shell $HM_PATH -A install --run 'home-manager switch'
to
nix-shell $HM_PATH -A install
The added shell hook will print some useful information and run
`home-manager switch`.
Before this path would point to the modules path. Using the project
root instead makes it possible to set `<home-manager>` to point to a
downloadable archive of Home Manager. This should make it
significantly easier to install and keep Home Manager up to date.
To match this change we also deprecate the Home Manager option
programs.home-manager.modulesPath
and instead ask users to use
programs.home-manager.path
