flake: update lockfile

[github-actions]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'nixpkgs':
    'https://releases.nixos.org/nixos/26.05/nixos-26.05.4937.8eeec934ae0d/nixexprs.tar.xz?narHash=sha256-LX2eBS9gFbXL5xUNBwp5mZ0tjnNvgiXoDnhz3ee1LzY%3D' (2026-07-14)
  → 'https://releases.nixos.org/nixos/26.05/nixos-26.05.5591.fd1462031fde/nixexprs.tar.xz?narHash=sha256-IX7j5TvSD7hYC9NZ8mgmq7pZ97mYyl24Qs4Tz4uyvO4%3D' (2026-07-19)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/db947814a175b7ca6ded66e21383d938df01c227?narHash=sha256-eynAfOmbmxJnkp7YewvCEbShNnnYJ9gLLqkzsYtBPeM%3D' (2026-05-31)
  → 'github:numtide/treefmt-nix/df3c0640565d04a0261253cdd89fce78ec50168a?narHash=sha256-47cxbcZODibHv3rELFQ9vZly0vUNkND/atn/U7HLeb0%3D' (2026-07-18)

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