home-manager: add activation sanity check of Nix
home-manager: add activation sanity check of Nix

This adds an empty `nix-build` command to verify that the user is
having a good Nix install. It also, as a side effect, will create the
necessary per-user `profiles` and `gcroots` directories.

Fixes #1246

rycee/home-manager,rycee/home-manager,rycee/home-manager