Update Nix flake inputs

[github-actions]

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

Flake lock file updates:

• Updated input 'fenix':
    'https://api.flakehub.com/f/pinned/nix-community/fenix/0.1.2471%2Brev-df3c2e78ec13418f85c1f26e77a50f865ec57d38/019ad905-65fc-7ba6-8d92-a0b2cf1c6ea6/source.tar.gz' (2025-12-01)
  → 'https://api.flakehub.com/f/pinned/nix-community/fenix/0.1.2511%2Brev-a3eaf682db8800962943a77ab77c0aae966f9825/019b78a8-f9ad-7faf-9a11-350b6ae3fcd9/source.tar.gz' (2026-01-01)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/d646b23' (2025-11-30)
  → 'github:rust-lang/rust-analyzer/a9026e6' (2025-12-31)
• Updated input 'nixpkgs':
    'https://api.flakehub.com/f/pinned/DeterminateSystems/secure/0.904649.313/019b4714-003c-7b50-a3aa-ffc368440845/source.tar.gz' (2025-12-22)
  → 'https://api.flakehub.com/f/pinned/DeterminateSystems/secure/0.904649.338/019bb3af-c7bf-7e96-a9c3-c69cfadb9af1/source.tar.gz' (2026-01-12)

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