Update nix

nix upgrade-nix

This might require to be run as root:

sudo -i nix upgrade-nix

-i to inherit the environment and have nix actually available in the $PATH

Sources:

• https://github.com/DeterminateSystems/nix-installer/issues/421
• https://github.com/DeterminateSystems/nix-installer/issues/508
• https://github.com/DeterminateSystems/nix-installer/issues/596