flake.lock: Update

Flake lock file updates:

• Updated input 'emacs-overlay':
    'github:nix-community/emacs-overlay/961ca255c7a1016667f1515f236008785064321a' (2023-09-20)
  → 'github:nix-community/emacs-overlay/8c25f71493cef83a6d41d8f46dc93bf3b5791c34' (2023-09-23)
• Updated input 'emacs-overlay/nixpkgs-stable':
    'github:NixOS/nixpkgs/5d017a8822e0907fb96f7700a319f9fe2434de02' (2023-09-17)
  → 'github:NixOS/nixpkgs/55ac2a9d2024f15c56adf20da505b29659911da8' (2023-09-21)
• Updated input 'gnu-elpa':
    'git+https://git.savannah.gnu.org/git/emacs/elpa.git?ref=main&rev=884336d14e8b62e4da80bdb3a16817013220039d' (2023-09-16)
  → 'git+https://git.savannah.gnu.org/git/emacs/elpa.git?ref=main&rev=570cc75a271daede2c198458572e393f08703ce7' (2023-09-20)
• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/970a59bd19eff3752ce552935687100c46e820a5' (2023-09-17)
  → 'github:NixOS/nixpkgs/5ba549eafcf3e33405e5f66decd1a72356632b96' (2023-09-19)