Install home-manager via `nix run` rather than `nix-build` then running
the activation script out of the store manually.
Co-authored-by: Cédric Barreteau <cbarrete@users.noreply.github.com>
Remove `stateVersion`, `username`, and `homeDirectory` as they can be
set in the configuration directly. Together with the previous commit,
this makes setting `stateVersion` explicitly mandatory.
Also replace `configuration` by `modules`, which expects a list of
Home Manager modules.
Also remove `system` which was made useless by #3028.
In the manual:
* Add chapter Nix Flakes
* Add links to sections of chapter "Using Home Manager"
In README.md:
* Remove section "Nix Flakes"
* Add manual reference at the buttom of section "Installation"