Update nix dependency to git revision that exists
Prior to this change attempting to build a fresh clone (on Linux) resulted in this error:
Updating git repository `https://github.com/AdminXVII/nix`
error: failed to load source for a dependency on `nix`
Caused by:
Unable to update https://github.com/AdminXVII/nix?branch=add-redox-support#65a419c8
Caused by:
revspec '65a419c85be86b00ab9b694f81eeb43116620f2b' not found; class=Reference (4); code=NotFound (-3)