[Nix-dev] Setting proxy environmental variables

[masterdeZign]

> You should only need to do a
>
>     4) nix-build -A system "<nixos>" --option use-binary-caches false

This step seems to require root privileges, because of "error: Nix
database directory `/nix/var/nix/db' is not writable: Permission
denied".

However, it is not a problem. I use sudo -E or sudo su + export the
http_proxy and https_proxy variables. But the problem persists,
because curl does not download anything.