flake.lock: Update

Flake lock file updates:

• Updated input 'merlin5_2':
    'github:ocaml/merlin/9dcffb9e998703f5f5d6e7c575c30cd822cea210?narHash=sha256-ZJFtPreWenLlXDokh3dOR%2Bb3LRuZJgs9%2B6r%2BtEx9/Vo%3D' (2025-01-10)
  → 'github:ocaml/merlin/1f73cadef1f906481236159adc77accaa9a28418?narHash=sha256-jAyw6kck7h4hOsHsUrILZgGol%2BQ/1Tc37phA/D6W46c%3D' (2025-01-31)
• Updated input 'nixpkgs':
    'github:nix-ocaml/nix-overlays/be7cfa6043ed31b17e4b86769c05825c62e55829?narHash=sha256-Ct6RqUtqIfazkg1X4o2FXWuYpw0A%2BOJsd3cFGtmXaqk%3D' (2025-01-09)
  → 'github:nix-ocaml/nix-overlays/a5124e087356e53d17eea757437daa9a02255249?narHash=sha256-1YDDo6qH9A%2BXIkPs0/gBpGzdept/kXv83GZgMfJRnsk%3D' (2025-02-01)
• Updated input 'nixpkgs/nixpkgs':
    'github:NixOS/nixpkgs/8e97141d59b87e2bf254cd0920be29955d45a698?narHash=sha256-h0hPzFp7iVhCqzBx%2BkJGdO/KmG8AkYRJ0jKxQ6%2Bdiug%3D' (2025-01-09)
  → 'github:NixOS/nixpkgs/3a228057f5b619feb3186e986dbe76278d707b6e?narHash=sha256-xvTo0Aw0%2Bveek7hvEVLzErmJyQkEcRk6PSR4zsRQFEc%3D' (2025-02-01)