The instructions to install nixos behind a proxy were not clear. While
one could guess that setting http_proxy variables can get the install
rolling, one could end up with an installed system where the proxy
settings for the nix-daemon are not configured.
This commit updates the documentation with
1. steps to install behind a proxy
2. configure the global proxy settings so that nix-daemon can access
internet.
3. Pointers to use nesting.clone in case one has to use different proxy
settings on different networks.
