flake.lock: Update

Flake input changes:

  • Updated 'nix': 'github:NixOS/nix/8a2ce0f455da32bc20978e68c0aad9efb4560abc' -> 'github:NixOS/nix/548437c2347159c4c79352283dd12ce58324f1d6'
  • Removed 'nix/lowdown-src'

Created by  Eelco Dolstra  on February 22, 2021
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