NixOS - Add proxy.nix to the default import
NixOS - Add xsel
NixOS - Add git-remote-hg

Beware, from behind a proxy, trouble.
I tried forcing git to use https:// over git:// (in .gitconfig) but this
did not work.
I finally use a fork of nixpkgs to adapt the default.nix (hard coding
https in url).
Then installed it as a user created the derivation.
Then nixos-rebuild switch reused the existing derivation.
alias - Add gfh for git fetch http-origin