flake.lock: Update

Flake input changes:

  • Updated 'nix': 'github:NixOS/nix/6529490cc10018d5191e50c482ac1180b96b1a3c' -> 'github:NixOS/nix/4ad5826a182f8eeb4cbc61e27d2128b08050b0d4'

Created by  Eelco Dolstra  on February 19, 2020
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