When you log in for the first time as a normal user, you should set up Home Manager. Once it's installed, you can use home-manager switch
as usual to manage the process, but to create the required configuration you need to evaluate it once directly from its activation package:
[you@somewhere:~]$ nix shell \
"github:impl/site-configs#homeConfigurations.${USER}@$(uname -n).activationPackage" \
--command home-manager-generation