deps: update flake.lock (#129)

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/2a2130494ad647f953593c4e84ea4df839fbd68c?narHash=sha256-Q82Ms%2BFQmgOBkdoSVm%2BFBpuFoeUAffNerR5yVV7SgT8%3D' (2025-07-08)
  → 'github:nixos/nixpkgs/6b4955211758ba47fac850c040a27f23b9b4008f?narHash=sha256-dPALCtmik9Wr14MGqVXm%2BOQcv7vhPBXcWNIOThGnB/Q%3D' (2025-07-19)

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Author: mrcjkb