Update nix to 0.25

[tmccombs]

No description provided.

[sharkdp]

Any particular reason for this change?

[tmccombs]

In fd I noticed that we now have both 0.24 and 0.25 of nix in the dependency graph. argmax requries 0.24.* and ctrlc (at least version 3.2.3) requires 0.25.0.

I suppose another way to avoid the duplicate dependency would be to change the dependency to something like

version = ">=0.24.2,<0.26"

also, nix probably isn't a very big dependency, so it might not matter too much.