flake.lock: Update

Flake lock file updates:

• Updated input 'neovim-nightly-overlay':
    'github:nix-community/neovim-nightly-overlay/8fa4fbe03aa7475441dd1b9c62e36feea083bfb0' (2024-04-14)
  → 'github:nix-community/neovim-nightly-overlay/3ad9c1eae733e9ba5cc73c6833e3d62dbc12df9e' (2024-04-21)
• Updated input 'neovim-nightly-overlay/neovim-flake':
    'github:neovim/neovim/a92822835521574710a830a7de0e692bf7517fb8?dir=contrib' (2024-04-13)
  → 'github:neovim/neovim/9e1bbb9813e0ea4e37f6325fe00e8f43617ef912?dir=contrib' (2024-04-20)
• Updated input 'neovim-nightly-overlay/neovim-flake/flake-utils':
    'github:numtide/flake-utils/4022d587cbbfd70fe950c1e2083a02621806a725' (2023-12-04)
  → 'github:numtide/flake-utils/b1d9ab70662946ef0850d488da1c9019f3a9752a' (2024-03-11)
• Updated input 'neovim-nightly-overlay/nixpkgs':
    'github:NixOS/nixpkgs/a0c9e3aee1000ac2bfb0e5b98c94c946a5d180a9' (2024-04-12)
  → 'github:NixOS/nixpkgs/fd16bb6d3bcca96039b11aa52038fafeb6e4f4be' (2024-04-20)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/a0c9e3aee1000ac2bfb0e5b98c94c946a5d180a9' (2024-04-12)
  → 'github:nixos/nixpkgs/fd16bb6d3bcca96039b11aa52038fafeb6e4f4be' (2024-04-20)