Updating nix
It seems like nix requires updating time to time. To update nix on a linux multi-user system:
$ su -
# nix-env --install --file '<nixpkgs>' --attr nix cacert -I nixpkgs=channel:nixpkgs-unstable
# systemctl daemon-reload
# systemctl restart nix-daemon
On a single-user system:
$ nix-env --install --file '<nixpkgs>' --attr nix cacert -I nixpkgs=channel:nixpkgs-unstable