Use proxy when installing, if proxy is set

[FriedrichAltheide]

It is currently not possible to install NixOS using calamares when behind a proxy, even when creating a custom iso with proxy environment variables set.

With the proposed changed: If a proxy is configured using the http_proxy, https_proxy as well as HTTP_PROXY and HTTPS_PROXY environment variables these environment variables are forwarded to the nixes-install command.