Using the `nix-env` command is far more robust. It also has the
benefit that if the per-user `profiles` and `gcroots` directories do
not exist then they will be created with the correct permissions.
Because of the second point this commit also removes the `mkdir` step
of the installation instructions.
PR #1239Closes#474, #948, #1091
It is safest to use the system install of Nix since that will be
compatible with the running nix-daemon and/or databases.
Also add a printout of the used Nix version in the activation script
when running in verbose mode.
Fixes#218.
